Last year’s winner Many Clouds will bid to emulate Red Rum by winning back-to-back runnings as 40 horses line up for the Grand National on Saturday.
Jockey Leighton Aspell, also successful in 2014 on Pineau De Re, is seeking a historic third consecutive victory in the famous race over 30 fences.
Coverage of the £1m race is expected to be followed by 600 million people worldwide.
Bookmakers say more than £150m will be wagered on the race.
The Aintree showpiece – run over nearly four-and-a-half miles – has been put back an hour to 17:15 BST, avoiding a clash with the afternoon’s football fixtures, in an effort to further increase audience figures.
History in the making?
Contenders have to negotiate obstacles such as Becher’s Brook and The Chair, and no horse has won back-to-back runnings since triple-winner Red Rum in the 1970s.
But Aspell believes Many Clouds, rated 7-1 favourite on Friday, can defy top weight for trainer Oliver Sherwood.
“I think he’s the worthy favourite. I would choose Many Clouds for his experience and class. He’s possibly still improving,” said Aspell, 39.
However, the jockey did concede that Silviniaco Conti, a seven-time winner of Grade One races at the highest level, had the capability to “make a mockery” of his 12-1 odds.
Who are the other leading contenders?
Punters will keep a keen eye on the weather, with the going at the track likely to be on the Good to Soft side.
Any rain would be considered a plus for last year’s Midlands National winner Goonyella, while the Jonjo O’Neill pair of Holywell and Shutthefrontdoor would prefer drier conditions.
Other leading contenders include The Last Samurai for trainer Kim Bailey, who triumphed 26 years ago with Mr Frisk.
One hopeful who has been significantly supported is the 2013 Cheltenham Gold Cup runner-up Sir Des Champs, trained by Willie Mullins.
Rated a 40-1 chance before the field was finalised on Thursday, his odds have halved since the booking of top jockey Ruby Walsh, who has won the race twice before – on Papillon in 1999 and Hedgehunter six years later.
However, Walsh must be passed fit to race by the doctor on Saturday morning after falling twice on Friday.
A National Velvet moment?
Ruby’s sister Katie is the sole female jockey in the National, and once again seeks to become the first female rider to triumph.
Katie, who achieved a best-placed finish of third in 2012 on Seabass, rides outsider Ballycasey for Mullins having been called up for the ride on Wednesday night.
As a child she watched the movie National Velvet, in which Elizabeth Taylor plays a young woman who rides the Grand National winner, but does not overplay her gender.
“I don’t see myself as a female jockey. I’m just a jockey,” said the 32-year-old Irish amateur.
“Loads of people say that if I win the National, I would be the first female to do so. I don’t see it like that. I just think it would be great to win.”
Are there safety concerns?
The National is famed as a unique test for horse and rider, but draws criticism from opponents, including animal welfare groups.
Officials believe modifications to the fences, and other alterations, introduced three years ago have helped improve safety.
Since the changes, there have been no fatal injuries to horses in the National itself, but there were three fatalities over the National fences on the first two days of this year’s Aintree meeting.
In Thursday’s Foxhunters Chase, Clonbanan Lad and Marasonnien did not fall, but were pulled up by their jockeys and later collapsed.
On Friday, Gullinbursti was put down after a fall at Becher’s Brook in the Grade Three Topham Chase.