The United States won their
fifth successive Olympic gold in the women's 4x400m.
Deedee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross finished more than three seconds ahead of nearest rivals Russia.
Jamaica beat off the challenge of Ukraine to take the bronze medal.
"American women's relay is very strong right now. They were always going to be tough to beat. It was always going to be tough for Great Britain to get a medal. Sanya Richards-Ross is one of the greatest 400m runners of all time and now she is a double Olympic champion at these Olympics. The US women did a great job."
Great Britain, who included Perri Shakes-Drayton and indivindual 400m silver medallist Christine Ohuruogu in their line-up, finished fifth.
The victory for the USA gives Felix her third gold of the Games following her wins in the 200m and 4x100m relay.
USA were the fastest team in qualifying, finishing a second a head of nearest rivals Russia, and they continued their good form.
Victory never looked in doubt after the first changeover and the remaining trio edged further clear of the rest of the field.
Richards-Ross, who won gold in the individual 400m, then carried the baton home, as they finished just 1.8 seconds outside the world record. The top six teams all ran a season's best time on the night.
Final results by Country
View full results for Women's 4 x 400m Relay
Trotter, Felix, Mccorory, Richards-ross, Baker, Dixon
Gushchina, Krivoshapka, Firova, Antyukh, Kapachinskaya, Nazarova
Day, Whyte, Williams, Williams-mills, Lloyd