Triple Crown winner American Pharoah has remained in his barn as heavy rain disrupted training for the Breeders' Cup at Keeneland.

The $US5 million ($A6.95 million) Classic on Saturday will be American Pharoah's final race before he retires to Coolmore's Ashford Stud.

Trainer Bob Baffert planned to send American Pharoah to the track for light exercise early on Wednesday after he arrived the day before from his home base in Southern California.

The 6-to-5 favourite in the Classic is expected to hit the track Thursday (Friday AEDT) when the weather improves.

Instead of working on the track, American Pharoah stayed warm and dry, strolling several times around the stalls in the barn enclosure with exercise rider Jorge Alvarez in the saddle.

The rain also kept champion mare Beholder, the 3-1 second choice in the Classic, in the barn for a second straight morning.

Golden Horn, the 4-5 favourite in the $3 million Turf, avoided the softened grass course for his gallop.

The Prix de l'Arc de Triomphe winner exercised on Keeneland's all-weather synthetic training track.

No rain is predicted for Friday, the start of the two-day Breeders' Cup, and there is only a 20 per cent chance of showers on Saturday.

If the forecast holds, the main dirt track will have enough time to drain should be fast for both racing days.

The turf course will undoubtedly retain moisture through the weekend, likely resulting in a soft or good rating.