World records for Australian 4x100m freestyle relay team and Hungary's Katinka Hosszu in 400m individual medley on first night in pool
The Campbells carried their squad to victory in the 4x100m freestyle relay at Rio 2016, with little sister Bronte snatching the lead from Dana Vollmer on the third leg and big sister Cate pulling away from Ledecky on the final down-and-back for a time of three minutes and 30.65 seconds.
The Aussies broke their own mark of 3:30.98 seconds set two years ago.
That wasn't the only world record to fall on a late night of swimming to kick off the Olympic Games. Shaking off repeated frustration on the sport's biggest stage, Katinka Hosszu of Hungary crushed the world record in the women's 400m individual medley to win the first Olympic medal of her career gold.
An earlier in the day, Great Britian's Adam Peaty has broken his own world record in the 100m breaststroke.
Also in the evening session, Japan's Kosuke Hagino ended American dominance in the men's 400m individual medley, while Australia's Mack Horton took down Sun Yang of China without giving his rival so much as a passing glance in the 400m freestyle.
The victories by Horton and the women's relay team capped an impressive first night for an Australian team that captured only one swimming gold medal in London four years ago. They have already eclipsed that total, with seven more nights to go.
Meanwhile, the powerful Americans managed three silvers but could not reach the top step on the podium.
Simone Manuel and Abbey Weitzeil did their best in the relay, giving the USA a lead at the midway point. But Bronte Campbell was more than a second faster than Vollmer, who had competed about an hour earlier in the semi-finals of the 100m butterfly, and Ledecky was no match for Cate Campbell, the world-record holder in the 100m free.
The USA had to settle for silver in an American-record time of 3:31.89, extending a gold medal drought in the event that goes back to the 1996 Atlanta Games. Canada claimed the bronze in 3:32.89.
Ledecky still has a strong shot at capturing five medals in Rio. She's favoured in the 200m, 400m and 800m freestyle events, and could take another gold in the 4x200m freestyle relay.
Hosszu, known as the 'Iron Lady' for her gruelling schedule, had an especially sweet triumph. She had captured nine medals — including five golds — at the world championships but had never won an Olympic medal. The Hungarian defiantly pumped her chest before breaking into a huge smile after her win on Saturday night.
She had led all the way and touched in 4:26.36, easily eclipsing the record of 4:28.43 held by China's Ye Shiwen. Hosszu even had time to turn toward the scoreboard and watch Maya DiRado of the United States touch in 4:31.15 to take the silver medal. Mireia Belmonte Garcia of Spain claimed the bronze in 4:32.39.
US swimmer Elizabeth Beisel, silver medallist at the 2012 London Games, finished sixth.
The 41st place finish that felt like gold for refugee swimmer Yusra Mardini
Hagino took gold in the men's 400m individual medley for Japan by holding off Chase Kalisz of the United States, becoming the first non-American to win the gruelling event since 1992.
Hagino and Japanese team-mate Daiya Seto raced away from the field on the butterfly and backstroke legs before Kalisz began to close the gap. The American surged past Seto on the breaststroke and set his sights on Hagino.
But the Japanese swimmer, who settled for bronze in this event at the 2012 London Games, held on to win in 4:06.05. Kalisz took silver in 4:06.75, while Seto grabbed the bronze in 4:09.71.