LAKE MEAD, Nev. — On this brisk, glittering morning, a flat-bottomed boat glides across the massive reservoir that provides Las Vegas its drinking water.

Federal scientists on board quickly stun dozens of fish in the water and scoop them up and soon will club the seven-pound common carps to death, draw their blood, snip out their gonads and pack them in aluminum foil and dry ice.

The specimens will be flown across the country to laboratories where aquatic toxicologists are studying what happens to fish that live in water contaminated with at least 13 different medications — from over-the-counter painkillers to prescription antibiotics and mood stabilizers.

More often than not these days, the laboratory tests bring unwelcome results.

A five-month Associated Press investigation has determined that trace amounts of many of the pharmaceuticals we take to stay healthy are seeping into drinking-water supplies, and a growing body of research indicates that this could harm humans.

But people aren't the only ones who consume that water. There is more and more evidence that some animals that live in or drink from streams and lakes are seriously affected.

Pharmaceuticals in the water are being blamed for severe reproductive problems in many types of fish: The endangered razorback sucker and male fathead minnow have been found with lower sperm counts and damaged sperm; some walleyes and male carp have become what are called feminized fish, producing egg yolk proteins typically made only by females.

Meanwhile, female fish have developed male genital organs. Also, there are skewed sex ratios in some aquatic populations, and sexually abnormal bass that produce cells for both sperm and eggs.

There are problems with other wildlife as well: kidney failure in vultures, impaired reproduction in mussels, inhibited growth in algae.

"We have no reason to think that this is a unique situation," says Erik Orsak, an environmental contaminants specialist with the U.S. Fish and Wildlife Service, pulling off rubber gloves splattered with fish blood at Lake Mead. "We find pretty much anywhere we look, these compounds are ubiquitous."

For example:

• In a broad study still under way, fish collected in waterways near or in Chicago; West Chester, Pa.; Orlando; Dallas; and Phoenix have tested positive for an array of pharmaceuticals — analgesics, antibiotics, antidepressants, antihistamines, anti-hypertension drugs and anti-seizure medications.

• That research follows a 2003 study in northern Texas, where every bluegill, black crappie and channel catfish researchers caught living downstream of a wastewater treatment plant tested positive for the active ingredients in two widely used antidepressants — one of the first times the residues of such drugs were detected in wildlife.