Chinese scientists say they have an apparently safe, virtually 100 percent effective male birth control pill, an American drug researcher reported yesterday after a tour of China.

American birth control experts were either skeptical that anything has been proved yet about the drug, or reserved judgment until they know more.

But the Rockefeller Foundation and the Population Council are starting studies with the Chinese to investigate the substance, the foundation's Dr. Sheldon Segal, a U.S. leader in contraceptive studies, disclosed.

A supply of the drug is now being shipped from the Chinese Institute of Materia Medica in Shanghai, and will be studied in animals at Rockefeller University "to see if a nontoxic dose can indeed stop sperm production" as the Chinese claim, Segal explained.

"The Chinese discovery is interesting and potentially important, but it's premature to draw any conclusions," said Segal, the Rockefeller Foundation's director of population sciences.

The Chinese male contraceptive drug is gossypol, a well-known chemical made from cottonseed.

Chinese doctors have used it on 3,000 men in the past four years and say they have found it 99.8 percent effective without adverse effects on sexual ability or fertility and without other serious side effects, said Dr. Stanley Kaplan, a research pharmacologist at Hoffman-La Roche Inc. at Nutley, N.J. Segal thought it more probable that the Chinese worked on the drug in animals for three years, and in men only in the past year.

Kaplan was the only scientist on a three-week tour of China in November. The other tourists were mainly American Planned Parenthood Federation volunteers and administrators and one physician, Dr. Judith Tyson.

"From what we heard," the Chinese pill "sounds safe, simple and highly effective," said Tyson, medical director of Vermont Planned Parenthood. "But so did" the birth control pill for women at first, "so I have a healthy skepticism," she added.

"Although I'm anxious to review their data, from what I heard I was very impressed," Kaplan said. "It sounded scientifically safe, and the Chinese appear optimistic."

"They say it's universally, completely effective. They've done a lot of animal testing as well."

At the same time, he added, the Chinese seemed to lack a complete, overall assessment of the pill's results, so we still have a lot to learn."

Some clinics reported some potassium depletion, he added, but not all did, "so the Chinese aren't sure if it was related to the drug." Also, he added, some men had some gastrointestinal effects, such as upset stomachs and nausea.

Kaplan emphasized that he is not a contraceptive expert or researcher and went on the tour as an individual, not a Hoffman-La Roche representative. He said his own special interests are pharmacokinetics -- what happens to drugs in the body -- and "drugs from naturally occurring substances," an area in which the Chinese have been highly active.

Both Segal and Dr. Alvin Paulsen of the University of Washington in Seattle -- a leading researcher on male contraceptives -- said they have heard of some "toxicity problems" with the drug.

"There are many unknowns about gossypol," Segal said. "Animal studies have shown a cumulative toxicity."

However, the Chinese may have purified the substance to minimize any problems, he added.

The Merck Index, a well-known pharmaceutical reference work, lists gossypol as a "poisonous" chemical with industrial uses and possible toxic effects on the lungs and nerves, including paralysis. But Kaplan said such effects would occur only in large doses, not the small ones in which the Chinese use the substance -- giving it to men daily for three months, with later maintenance doses.

Reported toxicity problems ended all discussion or investigation of the drug at a World Health Organization meeting in Geneva early last year, Paulsen said. But "we were told there were a large group of men involved in a clinical trial, and the details were just not available to us."

Dr. Gabriel Bialy, chief of contraceptive development at the National Institute of Child Health and Human Development in Bethesda, said he knew "nothing" about the pill and tended to be "reserved" in his opinion but still "optimistic," since "we cannot discount things just because we don't have knowledge of them."

Bialy said most American and European trials of male contraceptives have been disappointing. But Paulsen said his group has had two drugs -- Depo-Provera and testosterone, or male sex hormone -- under trial for as long as six to 10 months in human volunteers without any apparent harm, and "there will be a meeting next fall to consider whether we're ready for field trials.."