Tony McCoy has fallen from Synchronised ahead of the race. The horse galloped off done the course, but has been caught safely.
With some 600 millions TV viewers around the world, the Grand National is the biggest event in the horse racing calendar. ITV News Correspondent Harry Smith reports from Aintree Racecourse in Merseyside:
We are currently taking over £1.5 million an hour in #GrandNational bets. Shakalakaboomboom, Synchronised and Seabass leading the charge
Thanks for all the good luck tweets can't wait... Come on seabass!!!!
Katie Walsh, sister of Ruby, will be riding an Irish-trained nine-year-old called Seabass in her debut at the Grand National. If successful, she will become the first woman to ever win the race.
- The first Grand National was run in 1837 at the Great Liverpool Steeplechase at Maghull, three miles away from the present Aintree racecourse.
- The winner of that race was the aptly named Lottery.
- An estimated £300 million is staked on the race worldwide.
- The race is an energy-sapping four miles and four furlongs.
- During the early Grand Nationals, the obstacles included a stone wall located where the Water Jump is now.
- The Jockey Club bought Aintree in 1983, after 20 years of uncertainty over its future.
Today's race at Aintree will be the debut Grand National ride of star jockey Ruby Walsh's sister Katie. She will be riding an Irish-trained nine-year-old called Seabass.If successful, Katie would become the first woman ever to win the most famous horse race in the world.
She said: "I'm looking forward to it, it's a great opportunity and I might never have a ride in the race again, so I just want to enjoy the whole thing.
"As women riders we are not treated any differently and we wouldn't expect to be, but at some stage a woman is going to win a Classic on the Flat or the Grand National and I just hope it's me."
Millions of viewers worldwide are expected to follow the 165th John Smith's Grand National. Forty runners and riders will compete in the world's most famous steeplechase at Aintree this afternoon.
An estimated 600 million racing fans across the globe will be watching the race, which is scheduled to start at 4.15pm.
It will be the last time the legendary contest is broadcast in the UK by the BBC before moving to Channel 4 for the next four years.
The race will be worth £975,000 in prize money, making it one of the richest jump races in Europe