"Atrazine-exposed males were both demasculinized (chemically castrated) and completely feminized as adults," Tyrone Hayes of the University of California Berkeley and colleagues wrote in the Proceedings of the National Academy of Sciences.
The chemical had been shown to disrupt development and make frogs develop both male and female features -- termed hermaphroditism. This study of 40 male frogs shows the process can go even further, Hayes said.
"Before, we knew we got fewer males than we should have, and we got hermaphrodites. Now, we have clearly shown that many of these animals are sex-reversed males," Hayes said in a statement.
"Atrazine has caused a hormonal imbalance that has made them develop into the wrong sex, in terms of their genetic constitution."