Hundreds of athletes have travelled to a mid Wales town to take on the annual challenge of trying to beat a horse to the finishing line.
Can runners overcome horses in this madcap race?
This year's Man v Horse Marathon at Llanwrtyd Wells on Saturday has attracted more than 300 runners, all aiming to finish the 22-mile course ahead of their equine opponents.
No man has ever beaten the leading horse in the 24 years the event has been held.
The race is taking place on the same day as the Derby, and has a £24,000 prize for the winner.
The favourite to claim the cash prize is former Royal Marine Mark Croasdale, from Lancaster, who came within one minute of the leading horse and rider in the 2000 race.
Mr Croasdale, who has just returned from the Gulf, was among the runners lining up against around 40 horses at the start of the race at 1100 BST.
All have begun battling over the harsh terrain surrounding the smallest town in Britain - just as they have for every year since 1989.
Mark Croasdale has come nearest to claiming the prize
The money on offer is the biggest unclaimed prize in British athletics.
Bookmaker William Hill is offering odds of 20/1 that the horses will be beaten this year.
The firm is also offering 6/4 that the first relay team home will beat the first horse and rider to finish.
Even if the horses do win, the first runner will take home £600 as a consolation prize.
A special trophy will also be given out this year in memory of one of the UK's best-loved eccentrics Screaming Lord Sutch, who was the official starter of the race until his death in 1999.
The Lord Sutch Memorial Trophy - a specially created top hat and £250 prize - will be presented to the winner of a head-to-head showdown between a specially nominated runner and one of the horses.
The race was inspired by an 18th century tale of Guto Nyth Bran, who was reported to have raced against a horse in Cardiganshire and won.
Last year, the weather prevented a fast time, with the first horse finishing shortly after the two-hour mark.
Runners from Australia and America took on the challenge, but three horses crossed the finishing line some 15 minutes before the first runner.