The birds disappeared 25 years ago, then the grass and shrubs, for reasons that could only have made sense in China at the time.

The birds were systematically exterminated for eating farmers' seeds. Sparrows and bulbuls by the millions dropped dead of exhaustion as people beat pots, banged gongs and exploded firecrackers to keep them from resting on tree limbs.

Peking's greenery was uprooted in a later search-and-destroy mission designed to rid the city of insects by wiping out their breeding grounds. The pests had become a health menace because there were no longer birds to eat them.

Now, the first thing you notice about spring in Peking is how much it looks like winter--bald and birdless. A park in April is lasting testimony to the chain of man-made and natural disasters that have turned China's capital into an ecological basket case.

The city of 9 million, which has survived centuries of war, foreign occupation and political changes, is dying of self-abuse. It is choking on foul air and dust storms, drying up from an epochal drought and gagging on contaminated water and human filth.

Communist officials, whose crash industrialization and social neglect have upset Peking's balance of nature over the past three decades,have recently begun to realize the suicidal effects and become born-again ecologists.

The city has banned the construction of new heavy industry, ordered polluters to clean up their emissions and forced some factories to ration water.

Peking citizens once mobilized to kill birds and pull out every blade of grass are now exhorted to plant trees and shrubs.

Despite the best official efforts, however, Chinese and foreign experts believe that reversing Peking's environmental degradation will be as difficult as bringing back the songbirds.

An American Environmental Protection Agency (EPA) scientist visiting Peking two years ago conducted a spot air quality test and found pollution six times greater than is considered safe. He sent his findings to a friend at the U.S. Embassy here with a suggestion that he keep the data on hand "for the next time you have to justify your hardship differential."

A fresh glass of water upon analysis could pass for a test tube of toxic chemicals. The 1.6 million tons of raw sewage and industrial waste dumped daily into city streams pollutes underground water supplies with harmful amounts of mercury, cyanide, cobalt and benzene, according to Chinese reports.

Profligate industrial use of water plus the worst drought in 100 years have dried up a third of the city's wells, lowered the water table by three yards last year and caused an increase in the nitric acid content of drinking water.

A European diplomat who has suffered chronic intestinal and respiratory miseries for the past three years jokingly suggested that city officials post a sign at the airport saying, "living in Peking may be dangerous to your health."

Bicycle riders cover their faces with surgical masks and nylon scarves to keep from breathing coal dust. No one drinks the water before boiling it. Youngsters have few green spots for play.

In the 18th century, Jesuit missionaries based in Peking wrote of the "goodness" of the city's air and the meticulous cleanliness that made it "free of epidemics."

Over the past 200 years, Peking has evolved from a gentle town of royal families and mandarin scholars into a gritty industrial giant, which produces 80 percent of everything it consumes.

Like many Western cities, Peking grew without an eye to aesthetics or human need. Population has rocketed sixfold since 1949, and whole neighborhoods popped up without planning for sewers, roads or water.

The pressures of population in a city that offers each person 15 square feet of living space intensify the environmental problems.

Trying to expand their crowded living quarters, many Peking residents have added small wooden or brick structures to their houses, extending them into the narrow alleyways that serve as streets. The unauthorized building has caused a major sanitation problem by blocking garbage and night soil trucks from entering.

City officials are considering forming a sanitation police force to ensure the removal from public latrines of the 2,500 tons of night soil that accumulates daily and is supposed to be sent to farms as fertilizer.

Government regulators already have taken emergency measures to conserve dwindling water resources. In addition to rationing for several dozen industries, the city has installed water meters in some offices, schools and Army barracks to cut down usage.

With the water supply diminishing, the dangerous impact of pollutants has grown substantially. The official Health News Magazine, which issued the long list of drinking water contaminants, reported that the mercury content of fish in some areas exceeds government limits by 40 percent.

While cooking purifies water and food, it is more difficult to filter the air one breathes in Peking.

Most air pollution comes from the coal-burning stoves used for heating and cooking. According to the Peking Evening News, the ovens pump enough soot into the air each year to fill more than 6,000 railroad cars.

Tons of coal dust spewed out daily create what is known among foreign residents as "Peking lung"--chronic bronchitis. The EPA scientist who measured air quality said the soot is made of very small particles with pervasive effects on lungs.

On most days, smog is so thick it is impossible to see the hills that flank the city on the west. The coal residue mixed with what Chinese specialists say are high levels of sulfur dioxide, carbon monoxide and nitrogen oxide almost always make breathing difficult.

The difficulty multiplies every spring when vicious dust storms blow in sand from the Mongolian desert. The dust moves unobstructed through treeless sections of Peking, finding its way into window cracks, nostrils and lungs. Sometimes goggles are needed to see a few feet ahead.

As Chinese scientists and municipal officials have gained exposure to pollution problems and remedies in other parts of the world, they have moved with greater urgency to tackle Peking's woes.

Last February, the Academy of Sciences started the "urban ecosystem research project," which was called a "major scientific item" with the goal of making Peking more livable.

The project, which will recommend measures for cleaning up Peking, was described as "urgent and of great importance" because of the city's role as the nation's political and diplomatic center and because "it is a place that people throughout China look to as a model."

Even the city's greatest boosters would agree that, as urban models go, Peking needs work.