Zenyatta will put her 13-0 record on the line when she runs in the Breeders' Cup, most likely against the boys in the $5 million Classic.

A total of 166 horses, including 37 from Europe, were pre-entered Wednesday for the $25.5 million, 14-race Breeders' Cup world championships at Santa Anita on Nov. 6-7. That's down from last year's record of 180 horses.

Zenyatta was pre-entered in both the Classic, along with Kentucky Derby winner Mine That Bird and Belmont and Travers winner Summer Bird, and the $2 million Ladies' Classic.

But her connections are leaning toward the Classic on Nov. 7, pending the outcome of the 5-year-old mare's final six-furlong workout Saturday at Hollywood Park.

"There's definitely a preference for the Classic," trainer John Shirreffs said. "The Classic is obviously a big challenge, but if she's doing well and peaking towards that race, there's no reason not to go in that race."

Final entries and the post-position draw will be Tuesday.

Zenyatta has won all four of her starts this year. If she wins either race, she will surpass the mark of Personal Ensign, who won all 13 of her career races.

"I'm always amazed at how well she performs," Shirreffs said. "This is like a stopper on a baseball team. When you're not doing so well, you bring out Zenyatta and she lifts the whole barn."

Zenyatta won the Ladies' Classic last year on Santa Anita's synthetic Pro-Ride surface.

"The advantage of going in the Ladies' Classic is you have a great shot to win that race and it's a very large purse," Shirreffs said. "You have probably a better chance of winning there."

But if Zenyatta were to run against the boys for the first time _ and win _ she would make a strong case for Horse of the Year. Her Hall of Fame jockey Mike Smith has endorsed testing her against male competition in the Classic.

"She's going to be very, very tough to beat," said Alistair Donald of the International Racing Bureau. "If Zenyatta goes in the Classic, I fully expect her to be favored."

Horse racing's richest two days is returning to the same track for the first time in consecutive years. The track's surface, however, will prevent a showdown between Zenyatta and Preakness winner Rachel Alexandra, whose owner refused to run his sensational filly because he dislikes synthetic surfaces.

"You certainly have a responsibility to participate if you can," said Shirreffs, who has not been a proponent of California's mandated change from dirt to synthetic surfaces but has kept his stable based in the state.

Breeders' Cup president Greg Avioli was clear about which race he wants to see Zenyatta in.

"It's going to be a bigger story for the sport if she chooses to run in the Classic," he said. "It is the biggest of the Breeders' Cup races, it attracts the most attention."

The full field of 14 pre-entries for the 1 1/4-mile Classic include Irish standouts Rip Van Winkle and Mastercraftsman, who is more likely to run in the Dirt Mile; Arlington Million winner Gio Ponti; and Santa Anita Handicap winner Einstein.

Other Classic pre-entries include Pacific Classic winner Richard's Kid, trained by Bob Baffert, and Quality Road, trained by Todd Pletcher. Quality Road, an early favorite for the Kentucky Derby, missed the race because of injury.

Tim Ice, who trains Summer Bird, believes the Classic should decide Horse of the Year.

"Each sport has their championship game. If you don't compete in the championship game, then you shouldn't be a champion," he said. "This is the highlight of the year as far as all the great horses coming together. This is the stage you want to be on."

Last year, European horses won five Breeders' Cup races, including the 1-2 finish of Raven's Pass and Henrythenavigator in the Classic. This year's 37 European pre-entries are six more than last year, with Irishman Aidan O'Brien the only trainer with two Classic horses.

Goldolphin Racing, the Dubai-based stable of Sheik Mohammad, has a leading 17 pre-entries, including 16 trained by Saeed bin Suroor. Pletcher has 12.

Baffert will go for his third consecutive win in the Sprint with Zensational, and his second straight win in the Juvenile with Lookin at Lucky.

Ailing Hall of Fame trainer Bobby Frankel has three entries, including Proviso in the Ladies' Classic. He's been away from his stable much of the year with an undisclosed illness.

Zenyatta is one of a record 10 defending or former Breeders' Cup winners among the pre-entries. The others are: Ventura (Filly & Mare Sprint); Forever Together (Filly & Mare Turf); Maram (Juvenile Fillies Turf); Muhannak (Marathon); Desert Code (Turf Sprint); Goldikova (Mile); Midshipman (Juvenile); Conduit (Turf) and Red Rocks (2006 Turf).

If Zenyatta's owners opt for the Classic, her stablemate Life is Sweet was pre-entered in the Ladies' Classic, along with Alabama winner Careless Jewel and Beldame winner Music Note.

Composer Andrew Lloyd-Webber owns 4-year-old filly Dar Re Mi, who was pre-entered in both the $3 million Turf and the Filly & Mare Turf.

A maximum of 14 starters are allowed in each race, except for the Juvenile Turf, Juvenile Fillies Turf and the Dirt Mile, which are limited to 12.

Six of the 14 races were oversubscribed, including the Classic with 16 horses. For those races, fields are chosen based on three factors: BC Challenge winners; a points system; and a selection committee after the first seven starters have been determined.