WASHINGTON - A new study shows that male frogs exposed to the herbicide atrazine - commonly found in U.S. rivers and streams - can make a startling developmental U-turn, turning female so completely that they can mate with other males and lay viable eggs.

The study will focus new attention on concerns about atrazine, which is applied to an estimated 75 percent of American cornfields. Its manufacturer, the Swiss agricultural giant Syngenta, says the product is safe for wildlife, and for the people who are exposed to small amounts of it in drinking water.

In recent years, however, some studies have seemed to show that atrazine can drive natural hormone systems haywire in fish, birds, rats and frogs. In some cases, male animals exposed to the chemical developed female characteristics.

The study led by Tyrone Hayes, a professor at the University of California, was published in the Proceedings of the National Academy of Sciences. It showed an even starker transformation: Among a group of male African clawed frogs raised in water tainted with atrazine, he said, a fraction grew up to look and act like females.

"Ten percent of the chromosomal males become completely, functionally female," Hayes said in a telephone interview. "They can lay eggs (and) they mate with other males."

The offspring of those unions survived, he said, but they were all male, since both parents were genetically male. No female frogs were treated with atrazine in the study.

The other 90 percent of the exposed frogs retained some male features, Hayes said, but often showed signs of "feminization," including lower testosterone levels and fertility. When pitted head-to-head against males that had not been exposed to atrazine, the atrazine-treated males frequently lost out in competition for female frogs.

Hayes said the reason for these changes could be that atrazine, when absorbed through a frog's skin, helps produce an enzyme that converts an unusual amount of testosterone into estrogen.

Those high estrogen levels could then trigger developmental changes, he said.

Hayes, who has written several previous papers examining atrazine's effects on wildlife, said the findings should raise alarms about human health. Atrazine has been found in drinking water, particularly in agricultural areas of the country.

"It's a chemical ... that causes hormone havoc," Hayes said. "You need to look at things that are affecting wildlife, and realize that, biologically, we're not that different."

Those findings run counter to the U.S. Environmental Protection Agency's pronouncement in 2007 that atrazine does not cause problems in amphibian development. In a telephone interview Monday, Syngenta's principal scientist said that conclusion should settle concerns about the product's harm.

The EPA was "convinced that the question of whether atrazine affects frog sexual development," Tim Pastoor said. "The answer is no, it doesn't."

In 2006, the EPA made a similar finding that atrazine did not pose a danger to human health.

But last fall, the agency it would ask a panel of scientists to examine more recent studies of atrazine's effects. Steve Owens, an assistant EPA administrator, said the scientists would review Hayes's study and others, and report back early next year.

"We are looking very closely at the new science that has been developed," Owens said. "Right now, we can't say what the outcome of that study is going to be."

Atrazine is also a focus of scientists looking into a prominent case of gender-bending animal development: the "intersex" fish of the Potomac River. On Monday, Vicki Blazer of the U.S. Geological Survey - a leader in the effort to figure out what is making male bass grow eggs - said atrazine has been found in the Potomac, and in rural tributaries such as the Monocacy and Shenandoah rivers.

"It is one we're definitely concerned about," Blazer said.