Vicky Thornley, winner of the Trials in the past two seasons, got the time trials under way and promptly set the standard in the women's single scull with a time of 7:14.79.
Impressing behind her were three young rowers - Holly Nixon, Jess Leyden and Mathilda Hodgkins-Byrne - while Katie Greves, whose pairs partner Zoe Lee had to pull out through injury, was fifth-fastest on the 1,900m course.
London 2012 bronze-medalist Alan Campbell, an eight-times winner of the Trials, laid down the marker in what promises to be a hotly-contested men's single competition with the quickest time of 6:37.96.
Angus Groom, winner of the December assessment, Jonny Walton and Sam Townsend all posted times within two seconds of Campbell, with John Collins, Graeme Thomas, Peter Lambert and Jack Beaumont also in the mix.
Last year's lightweight women's single contest was fiercely fought between Charlotte Taylor, Imogen Walsh and Olympic champion Kat Copeland.
And those three look set to challenge for top spot again this year, with Taylor just getting the better of Walsh and Copeland in the time trials. Ellie Piggott, Brianna Stubbs and Emily Craig - all silver-medalists in the lightweight quad at last year's World Championships - made up the top six.
Will Fletcher, winner of the December Assessment, again set the pace in the lightweight men's single to come home in 6:55.50.
Sam Mottram, who made his senior debut at the 2015 World Championships, was second fastest, just ahead of Zak Lee-Green and two-times Trials winner Jamie Kirkwood.
The lightweight men's pair competition is a four-way tussle, with today's time trial determining the lanes for tomorrow's final.
Olympic silver-medalist Chris Bartley and Mark Aldred went fastest in 6:25.41, with Sam Scrimgeour and Jono Clegg in second.
Leading the way in the women's pair were Olympic, World and European Champions Helen Glover and Heather Stanning, who set a quick time of 6:53.23.
The chasing pack was led by four-time Olympian Frances Houghton, who has switched from sculling to sweep for these trials, and Olivia Carnegie-Brown.
GB Rowing Team Start graduate Karen Bennett and Melanie Wilson were third, just ahead of Jess Eddie and Polly Swann.
Eight reigning World Champions are contesting a stacked men's pair competition and they provided four of the top five placings in the final time trial of the morning.
Double Olympic champion Pete Reed and Constantine Louloudis were quickest in 6:11.90, with Alex Gregory and Moe Sbihi - winners of the last two GB Trials - just beating men's four World bronze-medalists Alan Sinclair and Stewart Innes to second spot.
London 2012 bronze-medalists George Nash and Will Satch were fourth, with Matt Gotrel and Paul Bennett fifth.
The action heats up this afternoon with lane racing in the semi-finals - they start at 3.07pm BST.