The United States collected its eighth swimming title with the women's 400-meter medley relay.
Beth Botsford put the Americans in front on the opening backstroke leg but South Africa's double Atlanta champion Penny Heyns took the lead from Amanda Beard on the breaststroke.
Angel Martino swam a storming butterfly leg to give the Americans an unbeatable lead and put the squad on world-record schedule, with Australia in pursuit.
Amy Van Dyken, winner of Tuesday's 100-meter butterfly, finished the job on the concluding freestyle section as the Americans won in 4:02.88 seconds _ outside China's world record 4:01.67.
Australia took the silver in 4:05.08 and China the bronze in 4:07.34.
America's female swimmers, predicted to go home empty-handed, have now won four gold medals.
"We've gotten closer as a team as the meet's gone by," said Martino, a former Furman swimmer. "Right before the relay we were doing relay starts and we were laughing and cutting up."
Martino, 29, and Van Dyken, 23, are the old women of the group. Beard, 14, and Botsford, 15, bring the average age down.
"We let them stand while we sit and save our energies," Martino said, laughing.
The foursome actually got a boost from Jenny Thompson, Catherine Fox, Kristine Quance and Whitney Hedgepath swimming the morning preliminaries. That group, which finished first also, will receive gold medals for their efforts.
"It's fair," said Van Dyken, who swam the freestyle leg in 54.17 to bring the U.S. home. "FINA (swimming's governing body) allows for that many swimmers. Our team decided to save the best for last. It's an advantage to that, but any team could do it. We're lucky to have four swimmers who are strong in the morning."
It was the seventh U.S. victory in the event and their fourth gold in four relays at these Games.
For Van Dyken it was her third gold medal of these Games, which equals an American record.
Having already won gold medals in the 100-meter butterfly and the 400-meter freestyle relay, Van Dyken swam a 54.17 final leg.
With the victory, Van Dyken tied a record held by 11 swimmers _ including Nicole Haislett, Janet Evans, Tracy Caulkins and Debbie Meyer _ for the most gold medals by an American woman in a single Olympic Games. Van Dyken is favored to pick up a fourth gold in Friday's 50 meter freestyle, her best event.
"That's my baby," she said.
After predictions of troubled Olympic waters for the U.S. team, the Americans have won eight golds, 18 medals total, one more than Australia, Germany and South Africa combined.
"I think we've exceeded the expectations of everyone," said Van Dyken. "I don't think we've exceeded our expectations. We knew if we worked hard we could come in here and be the underdogs with a really vicious bite."