European trainers John Gosden and Aidan O'Brien have watched their charges work in sodden conditions at Churchill Downs.

Plenty of rain has hit the Louisville circuit where Gosden is set to saddle dual Prix de l'Arc de Triomphe winner Enable in the Breeders' Cup Turf and Roaring Lion in the Classic.

Gosden watched as Enable went out for exercise under Frankie Dettori.

"The condition of the course is sodden, and it's pretty loose on the top," he said.

"I would say that on any other day's racing here they wouldn't be racing on the turf on Saturday.

"For all horses it will be a question of how they cope with it."

The forecast is for the rain to die down on Friday (Saturday AEDT) and for Saturday to be dry.

That scenario heartens O'Brien who expects the ground to quickly improve with its excellent drainage.

"We're pleased that the bulk of the rain came yesterday and today. I don't think there will be as much tomorrow and it is forecast to be dry on Saturday," O'Brien said.

"The turf course has an unbelievable drainage under it here, similar to the one at Adare Golf Club, and if any track will drain well, it's this one."

O'Brien's 15-horse team cleared quarantine on Thursday morning and jogged around the main dirt track on their first visit in heavy rain.

"The horses have been in the barn the last few days and I couldn't be happier with them," O'Brien said.

"They came out just now and trotted round once the opposite way, then turned round and did a gentle canter back to the winning post."

Mendelssohn was among the squad as he prepares for the Classic and the chance to redeem himself after an unfortunate experience in the Kentucky Derby.

"Donnacha (O'Brien) rode him this morning and was pleased. He has a very good constitution, this horse, and Ryan (Moore) is looking forward to him. It will be interesting," O'Brien said.