The Fox Hunters' Chase, the big race for amateur jockeys on the opening day of the Aintree Festival, was overshadowed by the death of Katie Walsh's mount Battlefront.
The 11-year-old, trained by Walsh's father Ted and owned by her mother Helen, died of a suspected heart attack after being pulled up at the 11th jump.
The race was the first at the meeting over the
new Grand National fences.
Walsh is due to ride Seabass in Saturday's Grand National.
Walsh wrote on Twitter: "Very sad to lose Battlefront today. We had many great days and he was a great teacher. He was a gent and I will miss him very much."
Earlier this week Walsh defended the Grand National over cruelty claims
and said that horses are treated "better than some children" despite recent deaths in the iconic race,
John Baker, Aintree and North West Regional director for Jockey Club Racecourses, said: "I would like to extend our sympathies to the Walsh family following the sad news.
"British racing is open that you can never remove risk from all horse racing, as with any sport. However, welfare standards are very high and equine fatalities are rare.
"The Grand National is woven into the fabric of British culture and, while the race is designed to be a unique and tough test, Aintree ensures it is as fair and as safe as possible.
"Nothing has greater importance to Aintree than the safety and welfare of the horses and riders. The racecourse constantly strives to improve this further."
Professor Chris Proudman, veterinary advisor to Aintree racecourse, said: "The horse has been sent to undergo a post-mortem examination at the University of Liverpool.
"From a veterinary perspective, no other serious problems were reported with the 95 other runners racing today."
Five horses fell in the Fox Hunters' Chase, over two miles and five furlongs of the National course, although there were no serious injuries reported.
Fourteen of the 25 starters finished the race, which was won by 100-1 shot Tartan Snow.
The changes to the fences, which now have a core made of more flexible plastic, rather than wood, and are covered by the traditional spruce,
were trialled at Aintree
in December 2012
Jockeys who rode in Thursday's race had reported positively about the redesigned fences.
Jamie Hamilton, 18, who rode the winner Tartan Snow, said: "It's the first time I have ridden over them but they seemed spot on."
Fellow amateur Sam Waley-Cohen, the rider of unplaced Cottage Oak, told BBC Sport: "They are still big obstacles and are a test for horse and jockey."
Waley-Cohen, who has been a three-time winner in races over the National fences, added: "If the horse can jump, they are great."