The USA destroyed the world record in taking Olympic gold in the women's 4x200m freestyle as Great Britain finished out of the medals in fifth.
The Americans came home in a time of seven minutes 53.42 seconds, over two seconds inside the previous best mark.
China claimed silver with Germany holding off Australia for bronze.
Britain had won their heat and Melanie Marshall swam a strong opening leg to leave the team lying second but their challenge faded on legs two and three.
The USA's first-leg swimmer Natalie Coughlin went immediately into the lead and her team-mates Carly Piper, Dana Vollmer and Kaitlin Sandeno were thereafter never troubled.
Marshall had earlier pulled out of the 100m freestyle to concentrate her efforts on the relay and things looked promising as she handed over to Georgina Lee in second.
But Germany, China and Australia were strong with their middle orders and when Caitlin McClatchey handed over to veteran Karen Pickering for the anchor leg, Britain's hopes had gone.
Pickering, competing in her fourth Games, did make some headway but failed in her quest for her first Olympic medal.