ATHENS, Ga. --- A University of Georgia study will be aimed at determining the effect estrogens in the Oconee River have on fish.

In other American rivers, researchers have detected measurable quantities of the chemicals, called environmental estrogens. Until now, no one has tried to measure their impact in Georgia rivers.

In past studies, male fish exposed to even small levels of environmental estrogens -- 5 or 6 parts per trillion -- begin producing female egg proteins, and females begin producing fewer viable eggs.

This summer, UGA researchers will begin administering small doses of chemicals to minnows, then measure how different levels are changing sexual characteristics and ability to reproduce.

In the fall, they will begin to put fish in the Oconee River upstream and downstream of outlets of Athens wastewater treatment plants. They will also trap wild fish from the same spots and look for evidence of estrogen exposure.

Some of the chemicals are natural sex hormones produced in female ovaries, but scientists also have found synthetic estrogens, used in birth control pills and for hormone replacement therapy, and chemicals that affect humans and other animals in the same way estrogens do.

The researchers suspect that even fish exposed for a short time -- especially at critical stages of development -- could have effects that might not show up until the fish are adults and try to reproduce.

"It's fairly predictable, if you look in any area that receives wastewater effluent," said Robert Bringolf, a professor of fish biology and ecotoxicology in UGA's Warnell School of Forestry and Natural Resources.

In UGA's fishery labs, graduate student Kristen Kellock is working on the first part of the study, growing hundreds of fathead minnows, a species common across much of the country.

In a third stage, the researchers plan to measure levels of some of the chemicals in river water.

Mr. Bringolf and Ms. Kellock won't be surprised to find relatively high levels of environmental estrogens in the North and Middle Oconee rivers.

Mr. Bringolf wants to find out how these chemicals are affecting river ecology and whether harmful effects are limited to relatively few fish or spread to other aquatic life.