In its aggressive strides to improve the quality of life, New York City has drastically curtailed its crime rate. It has overhauled its chaotic school system. But in one area, success is elusive: The city's rats remain as bold and showy as ever, darting through well-lighted subway stations as blase New Yorkers watch and scurrying through its public parks at will.

Confounded in previous tries to control the rat population, the city has profoundly shifted its approach in recent few years in an experiment that is being watched by cities across the nation. Instead of waiting for residents' complaints and responding with rat poison and baits, officials want to tackle the root cause of infestations. They hope to reduce the number of rats by curtailing the food supply — exposed or easily accessible garbage — and controlling the clutter and debris where rats thrive.

The jury is still out on whether the new strategy is working. The number of pest-control complaints has nearly doubled since Mayor Michael R. Bloomberg took office in 2002, surging to 32,160 in the fiscal year that ended on June 30, the highest in years. But the number of rat bites, which disproportionately affect children and which are considered especially worrisome because rats can spread disease, is at its lowest level in decades. The disparity has mystified some experts.

Officials attribute part of the increase in complaints to the inception of the city's 311 hot line in 2003, but they concede that the problem is broad and stubborn. Surveys by the Census Bureau show the rate of rodent infestations — 28.7 percent of rental units and 7.6 percent of owner-occupied housing last year — has remained fairly stable since the late 1990s.

In the last fiscal year, the number of exterminations plummeted, but city officials attribute part of that drop to the improved handling of duplicate complaints. However, it can take months for the city to respond to a rodent complaint, a problem that has been cited in several audits by the city comptroller.

One part of the new effort involves collecting information on computers, block by block and building by building in the most rat-infested areas of the city. Another element is a public education campaign with colorful posters and slogans like "Feed a Pigeon, Breed a Rat" and "Help New York City Send Rats Packing." The city also conducts classes for many workers called the Rodent Control Academy, using a federal grant.

In addition, each Tuesday, representatives of 22 agencies — including the Buildings, Health, Parks and Sanitation Departments, the Metropolitan Transportation Authority and the Housing Authority — gather in Lower Manhattan to discuss who should handle rat sightings in places like sewer drains, alleys and playgrounds. The group, known as the citywide rodent task force, was formed in 2000 under Mayor Rudolph W. Giuliani and has been expanded under Mr. Bloomberg.

The most effective strategies for rat control have been known for a century but have rarely been followed with rigor, according to Stephen C. Frantz, a retired State Department of Health official and an authority on animal-borne diseases.

"The key to success was environmental management that focused on eliminating food sources, water and harborage," he said. "Today, we place far too much reliance on poisoning."

Complicating the problems, rats are developing resistance to many of the poisons used on them.

The Norway or brown rat, the species prevalent on the East Coast, has been around since the Colonial days, but systematic efforts to control the pests did not begin until the late 19th century, with the hiring of rat catchers paid by the rat. The development of powerful rodenticides in the 1940s prompted widespread extermination efforts. New York City officials started such a campaign in 1948.

Later, New York was one of 65 localities that received federal grants from 1972 to 1981 under a program that emphasized public education, sanitation, code enforcement and, to varying extents, poisoning, according to Jerry M. Hershovitz, a longtime official at the Centers for Disease Control and Prevention who oversaw the federal program.

"Unfortunately, since 1981, when the federal program ended, there's been a resurgence of the rodent problem in many communities," said Mr. Hershovitz, who retired this year. "Today, most rodent control programs are complaint-oriented and are limited to poisoning and, sometimes, trapping. Undoubtedly, rodents are taking advantage of the situation."

New York's rat problem gained new attention under Mr. Giuliani, who doubled spending on pest control in 1997 under pressure from politicians in northern Manhattan, which has historically been a major theater in the war on rats.

Bill Perkins, who represented central Harlem on the City Council from 1998 to 2005 and was elected last month to the State Senate, led a committee that convened a "rat summit" at Columbia University, held seven hearings and produced a report in 2001 that called for lessening the use of poison and improving trash removal and sealing cracks and holes in building foundations that allow rats to move about.

That preventive approach, known among specialists as integrated pest management, has gradually become the city health department's official policy, particularly because of increasing safety concerns about poisons and the resistance rats are beginning to show to the poisons.

The city's first pilot project based on that approach used a federal grant to control rats in a 48-block area in Bushwick, Brooklyn, starting in November 2001. The area was inspected for conditions that encourage rats: loose garbage, overgrown lots and holes that let them get into buildings.

The city hired two neighborhood organizations to hand out brochures and put up posters. The city cleared litter and debris from parks, schools, playgrounds and city-managed apartment buildings and other public properties. Rat-resistant trash cans were given away. Within a year, more than half of the properties met inspection standards, up from one-third.

Using city money, the Bloomberg administration expanded the program in August 2003 to cover 1,500 blocks in central Brooklyn, the southern Bronx and northern Manhattan. The rat populations dwindled in those neighborhoods, but the program, the Mayor's Rodent Control Initiative, ended last December after a series of building-by-building inspections was completed. Some fear that bad human habits may return, and, with them, the rats.

With limited resources, the health department is now trying to apply integrated pest management by collecting better neighborhood data.

"Rodents don't just live on one property," said Jessica Leighton, an epidemiologist who has been the deputy commissioner for environmental health since last year. "They're in a community."

One of her deputies, Edgar R. Butts, a plant physiologist, said: "You can bring a trainload or boatload of rodenticide into the city. But as long as you have food and harborage, you'll have rats."

The Bureau of Pest Control Services has a staff of about 235, which includes 43 inspectors, known as sanitarians; 24 exterminators; and 111 lot cleaners. The department spends about more than $8 million annually on rat control, a number that has fluctuated over 20 years.

Michael Mills and Eric Han, both sanitarians, are putting into practice a strategy of rat surveillance, known as indexing. Using maps and property information downloaded onto tablet computers, they look for six "active rat signs": tracks, active runs (streak marks created when rats run along walls), fresh droppings, gnawing, visible holes and "live rats seen." (The last is, mercifully, rare.) Each characteristic is recorded on a scale of zero to three.

On a recent walk through the Bedford Park neighborhood in the Bronx, the two men pointed out relics of private efforts, like abandoned bait stations and haphazardly applied patches of concrete. Some property owners even cordon off their yards with sheet metal in a usually futile effort to prevent rats from entering.

Why the rats remain is no mystery, given the abundance of waste New Yorkers leave behind. In an alley next to an apartment building were two exposed trash cans. Inside one was an empty can of Chef Boyardee spaghetti and meatballs, with a residue of sauce.

At another building, the workers found a series of freshly dug burrows at the base of some yew bushes in a concrete, elevated planter that ran along the front. The planter was littered with paper, a discarded soda cup and other trash. A white foam food container was perched at the top of one burrow, apparently dragged there by a rat.

The side yard of a 25-unit building on Valentine Avenue held piles of trash, construction supplies and other clutter. In the corners of the lot were the soft, black pellets: rat droppings. The building has new owners who have hired private exterminators and a new superintendent, a property manager said.

The data will eventually help health officials to prioritize blocks with improperly stored garbage or hazardous amounts of debris; to use rodenticides more selectively; and to better inform residents about how to dispose of garbage. Rats bore into plastic bags effortlessly and can even gnaw through thick plastic trash can lids. Metal cans are ideal, but sanitation workers do not like them because they are heavier and have been linked to injuries. At the least, residents are urged to cover their cans.

While the city does not know if its rat population is increasing or declining, the complaints are continuing to roll in.

Owners of private property are responsible for controlling rat infestations on their premises. If inspectors responding to a complaint — most often from renters — find signs of rats, they issue a letter giving the owner five days to correct the problem. If the problem is still there on a follow-up inspection, the department sends in exterminators and lot cleaners, then bills the owner for the work.

The surge in complaints keeps the exterminators busy. On a recent morning, two veteran exterminators, Larry J. Adams and Harold B. Pou, parked a city van in front of an apartment building in Brooklyn. The front yard of a building on Bushwick Avenue was perforated with rat burrows, indicating a severe infestation.

Using a rubber tube, they poured packet after packet of a rodenticide, Talon-G, into the burrows. Each packet contains 25 milligrams of blue-green pellets of brodifacoum, an anticoagulant that is highly toxic to rats and leads to fatal bleeding.

"We've complained so many times," said Manuel Matias, 24, a resident who peered out of a third-floor window to watch the work.

Outside another apartment building, on Eastern Parkway in Crown Heights, Brooklyn, a rat carcass was in plain view. The yard was riddled with burrows, and a superintendent said the owners planned to pave it.

Mr. Pou marveled at the yard. "This is what I call a golf course: 18 holes," he said. "We've been here many times, and we just can't seem to beat them."

Copyright 2006 The New York Times Company