LOUISVILLE, Ky. — "Is that the one-horse guy?" a visitor asked as he walked past Tom McCarthy's rented stall on the backstretch at Churchill Downs.

Indeed, it was.

The 75-year-old owner-trainer signed a few autographs yesterday under the watchful gaze of his big gray colt, General Quarters, who cost him all of $20,000.

McCarthy, a retired high school principal and biology teacher from Louisville who does everything but ride the horse himself, faces the biggest exam of his life in today's Kentucky Derby.

"How cool would it be if he won?" wondered Hall of Fame trainer Bob Baffert, who saddles early co-second choice Pioneerof the Nile in the 1 1/4-mile race.

That probably would be more than fine with scores of Derby fans still trying to get past last year's awful ending when filly Eight Belles finished second to Big Brown, then went down on two broken ankles and had to be destroyed.

Come race day, McCarthy won't be one of the trainers hanging out in the box seats and accepting good wishes. Instead, he'll work in the barn until it's time to saddle General Quarters, who gave him his first and, so far, only victory in a Grade 1 race — the Blue Grass Stakes.

"We're here to do a job, and he's the only one I really need to be with on Derby Day," said McCarthy, who watched from the rail at his first Derby in 1955 and has attended nearly every one since. "I've got more confidence in him than I have in myself."

McCarthy would be just fine, too, if forecasts calling for rain during the afternoon turn out to be right. General Quarters has never raced on an off-track, but his pedigree suggests he could run well in the slop.

"He gallops over the mud almost like he does over the dry (surface)," McCarthy said after the colt galloped yesterday morning. "Once he gets moving, he's like a big train. He's hard to stop."

Just six horses in the full field of 20 have run on an off-track. Among those who haven't are I Want Revenge, Pioneerof the Nile, and his co-second choice, Dunkirk.

"The only thing I could ask for is better weather and a fast race track," said Jeff Mullins, who trains Wood Memorial winner I Want Revenge.

Mullins comes into his sixth Derby with a seven-day suspension looming. Even if I Want Revenge wins, the California based-trainer must be off the Churchill Downs grounds by midnight tonight.

It's punishment for administering an over-the-counter medication to one of his horses in a security barn just before a race in New York last month.

I Want Revenge and 19-year-old jockey Joe Talamo will break from the No. 13 post. The favorite has won the Derby the last two years and three times in the previous five years.

The California-based colt left that state's synthetic tracks to try dirt in New York and is coming off eye-catching victories in the Gotham and Wood.

But he's among eight colts in the Derby who've run mostly on artificial surfaces, making it tricky for bettors to handicap how horses not used to the stuff will fare if mud starts flying in their faces.

"It's hard to evaluate how good they are," four-time Derby winner D. Wayne Lukas said. "Some of them could adapt to this beautifully and others bomb, so it makes it a nightmare to handicap. There could be a $50 or $60 (win) payoff pretty easy."

Friesan Fire, the early 5-1 fourth choice, won and finished third in his two races on wet tracks. He hasn't raced since winning the Louisiana Derby on March 14, the longest layoff of anyone in the field.

"If we get the right trip, we'll be right there," trainer Larry Jones said.

Jones finished second in the Derby with Eight Belles last year and Hard Spun in 2007. He will be looking for his first victory in his last attempt at winning the Derby since he plans to retire later this year.

Asked about his ideal result, Jones said with a laugh:

"We win. We win in Baltimore and then Belmont. What a way to go out!"

Dunkirk lacks experience on two fronts: He's made just three career starts and he's never run in the mud.

The Florida Derby runner-up didn't race as a 2-year-old because of minor physical problems. No horse since Apollo in 1882 has won with such little seasoning.

Dunkirk's trainer, Todd Pletcher, also saddling long shots Advice and Join in the Dance, will attempt to end his 0-for-21 streak in the Derby.

"I don't think we want to see a 'good' or 'muddy' track," he said. "That won't help my horses."