WASHINGTON -- In findings that renew a debate over whether chemicals or other environmental factors influence sexual development, U.S. researchers said some rural men have lower sperm counts than their big-city counterparts.

Men living in rural Missouri had lower sperm counts than men living in New York, Minneapolis and Los Angeles, the researchers said in a report released yesterday.

It joins a collection of studies that have shown conflicting results on whether a man's sperm count is affected by where he lives and the kind of things he is exposed to.

"What we found is that men from Columbia, Mo., which is an agricultural community, have significantly lower sperm density and sperm motility [movement] related to the three urban centres we looked at," said Shanna Swan, who led the study.

Dr. Swan, an epidemiologist at the University of Missouri in Columbia who has devoted her career to this issue, believes agricultural chemicals may be to blame. "A lot of agricultural chemicals are used around here and are known to get into drinking water," she said.

Writing in the December issue of Environmental Health Perspectives, Dr. Swan and colleagues across the United States said they studied 512 couples, interviewing the men and taking blood and semen samples.

Semen quality was equally high in Minneapolis and New York, and slightly lower in Los Angeles. Men from around Columbia had sperm counts and quality that were significantly lower than men from any of the three cities.

Dr. Swan's team is now looking for clues in the men's blood. The U.S. Centers for Disease Control and Prevention is analyzing the blood for chemicals to see whether there are differences between the city and country dwellers.

The team also will examine hormone levels. Besides chemicals, stress is a factor that could affect sperm, Dr. Swan said.

"In another study [looking at stress], the only thing related to semen quality was loss -- the death of a close family member or divorce," she noted. "One hypothesis is that population density itself might be increasing semen quality. There are some indications that in some animal species that are crowded, the dominant males become more fertile."

But most compelling to Dr. Swan was the idea that certain chemicals can act as endocrine disruptors, changing the way hormones work in the body.

They include pesticides such as dichloro diphenyl trichlorethane (DDT), and industrial chemicals such as polychlorinated biphenyls (PCBs). Such "gender- bending" chemicals are blamed for causing genital abnormalities in animals such as frogs and alligators.

Fears that humans could be suffering similar, albeit more subtle, effects emerged in 1992, when Danish researchers reported that sperm counts were falling around the world.