The top two swimmers from each event plus four more in both the 100 and 200 freestyles qualify for the 2012 Olympics, set for July 27-Aug. 12.
"It was just so amazing to swim in front of a crowd of thousands of people and I was just happy to make it back, honestly, for a second swim at night because I'm really focused on the 200 breast on Saturday," Kropp said. "It was great just getting a taste of what swimming at night in front of all those people was going to be like."
Kropp, a two-time All-Area Girls' Swimmer of the Year, also swam in the 400-individual medley on Monday, but did not make the top-eight cutoff for the semifinals, as she finished with the 13th-best time (4:45.77).
Kropp previously competed at the 2008 Olympic Team Trials, where she finished no higher than 28th in any of her four events.
"Last time I was just petrified," said Kropp, who will next compete in the 200-individual medley prelims today. "Now, not just having the experience of trials before, but all the international meets that I've had the opportunity to go to since then has really allowed me to kind of distance myself from the enormity of the meet."
The schedule for the meet, which continues through Saturday, features preliminary events beginning at 10 a.m. CDT and semifinals and finals sessions at 6:45 p.m. CDT.
Campbell, a La Cañada graduate who is coming off a senior season in which she won her fourth CIF Southern Section Division I 500-yard freestyle title and second straight All-Area Girls' Swimmer of the Year accolade, competed in the 400-meter freestyle prelims on Tuesday.
She recorded the 61st-best time of 4:19.85, but will also compete in the 800 freestyle on Saturday.
Seo, a senior-to-be and the reigning All-Area Boys' Swimmer of the Year who led Crescenta Valley High to the CIF Division II championship in May, made his debut on Monday, placing 91st in the 400-individual medley.
He will continue to compete on when he takes part in 200 butterfly prelims today.
This is the second trials for 2009 Crescenta Valley graduate Yumi So, who had the 89th-best time in the 100 backstroke on Tuesday after being disqualified in the 100 butterfly on Monday.