LOUISVILLE, Ky. — All week, the chatter along the Churchill Downs backstretch has been about Big Brown running away with the Kentucky Derby.

The talk ends today and the real running begins in 1 1/4-mile Derby, where the unbeaten 3-year-old colt has plenty to prove, history to buck and perhaps even a wet track to overcome.

Big Brown is the 3-1 morning-line favorite in a full 20-horse field that includes Santa Anita Derby winner Colonel John and Eight Belles, who will try to become the fourth filly to win the Run for the Roses.

Toss in Louisiana Derby winner Pyro, Blue Grass winner Monba, Wood Memorial winner Tale of Ekati and Arkansas Derby winner Gayego, and the race could be more wide-open than many expect.

"His favoritism is clear," Derby oddsmaker Mike Battaglia says, referring to Big Brown. "But is it really clear he's that good?"

So far, Big Brown has looked the part of a champion, and trainer Rick Dutrow Jr. has voiced his confidence in the 3-year-old colt for more than a month. The Florida Derby winner has won all three of his races by a combined 29 lengths.

"I feel we have the best horse in the race, the fastest horse," Dutrow said yesterday morning outside his barn. "He's giving me every indication that he is sitting on a big race."

Another dominant win by Big Brown would renew hopes that there could be a Triple Crown champion for the first time since Affirmed swept the Derby, Preakness and Belmont Stakes in 1978.

Colonel John, the 4-1 second choice, has never been worse than second in six career starts — all on synthetic tracks made of wax-coated sand, fibers and recycled rubber.

That raises the second-most compelling question in this Derby: Can he make the transition to dirt?

"I've liked what I've seen since I got here and I have no reason to change my opinion," trainer Eoin Harty said.

And what about the filly Eight Belles, 4 for 4 this year, taking on the boys for the first time? It's been nine years since a filly ran in the Derby, with Winning Colors the last to win it in 1988.

"We think she belongs," Eight Belles owner Rick Porter said. "Right now, you don't know how good Big Brown is."

Only three favorites have won since Spectacular Bid in 1979, including Street Sense last year. Giacomo, at 50-1 odds, won in 2005.

Overnight thunderstorms could leave the racing strip a mess today. The rain was expected to end at about 3 a.m., Hawai'i time, today, meteorologist John Gordon of the National Weather Service in Louisville said. More rain was possible in the afternoon until about noon, just as the race is scheduled to start.

A wet track could be good news for a bunch of Derby long shots. Cool Coal Man, Visionaire, Smooth Air and Denis of Cork have won over sloppy tracks. Pyro ran second in the Breeders' Cup Juvenile at muddy Monmouth Park. Tale of Ekati, Big Truck and Z Humor hate the mud.

There's a bit of Derby history being challenged, too.

Big Brown and Colonel John, for example, are among six horses with just two preps this year. The others are Court Vision, Monba, Tale of Ekati and Recapturetheglory. Since 1947, only Sunny's Halo in 1983 and Street Sense have won the Derby off two preps. Big Brown is also looking at this: The last Derby winner with three career starts was the filly Regret in 1915.

Big Brown's fate could be decided at the start. A clean break and fast start are imperative for a horse on the far outside. Since 1900, the only Derby winner from the No. 20 post was Clyde Van Dusen in 1929.

"It might be too far out there, but we were willing to accept that challenge," Dutrow said. "He usually breaks good. He usually puts himself right in the game."