Kentucky Derby favourite Justify will jump from barrier seven of 20 in the 144th Run for the Roses, the first jewel in American thoroughbred racing's Triple Crown.

Unbeaten in three starts, the Bob Baffert-trained Justify is at 3-1 for the famous race at Churchill Downs on Saturday (Sunday AEST).

He will attempt to become the first horse since 1882 to win the Kentucky Derby without having raced as a two-year-old, known as the Curse of the Apollo in honour of the last horse to achieve that feat.

Mendelssohn, winner of the UAE Derby by 18 lengths, will start from gate 14 and is the second favourite at 5-1.

The Aidan O'Brien-trained winner of the UAE Derby, Mendelssohn arrived from Ireland on Monday and will have a work-out at Churchill Downs on Wednesday (Thursday AEST).

"I'm happy, I think everybody should be happy with their post," Baffert said.

"I wanted to be in the middle or outside. We didn't want the one hole that's for sure, so I was relieved when I saw him in seventh.

"It is one of the toughest Derbys I've ever seen.."

Baffert has trained four Kentucky Derby winners including American Pharoah who won the triple crown in 2015.

While the Curse of Apollo may be a concern for the superstitious, on the other side of the coin favourites have won the past five Kentucky Derbys and seven of the past 11.

Magnum Moon, winner of both the Arkansas Derby and Rebel Stakes, was set as third favourite at 6-1 and has barrier 16.