Saturday night in Las Vegas, Randy Couture will step into an eight-sided ring surrounded by a steel cage and stare down Brock Lesnar, a 31-year-old former NCAA wrestling champion who is six inches taller, at least 40 pounds heavier and 14 years younger. Mr. Couture, who is 45, will then have 25 minutes to knock his opponent unconscious, force him to quit or control him thoroughly enough to win by decision.

Pay-per-view-television experts say it's possible that Saturday's fight, at the MGM Grand in Las Vegas, could draw more than a million buys at $44.95 apiece, which would match the sport's record and rank higher than all but two boxing matches from the last four years.

Marc Ratner, a longtime Nevada boxing official who is now an executive for the UFC, the sport's largest governing body and this fight's sponsor, explains the sport's appeal this way: "If you're in the middle of a street somewhere, and guys are playing pickup basketball to one side, and playing football on the other corner, and playing baseball, and a fight broke out, you'd automatically gravitate to the fight. It's something in our soul, our DNA."

For more than a century, this primal drive to watch people fight has been satisfied by boxing, a traditional sport that tests combat ability along just one dimension: punching. Mixed martial arts permits a much wider range of striking (kicking and open-hand hits are permitted, as are most blows below the waist) as well as submissions, takedowns, and ground fighting, testing ability of all three. Fighters combine elements of martial arts including muay Thai, a form of boxing that allows the use of the knees, elbows and feet; sambo, a Russian variant of judo; and even karate. To be effective, a fighter has to develop a well-rounded game. Without extensive training, even a world-class wrestler or boxer can be thrashed.

"There's a technical and tactical aspect to this sport," says Mr. Couture, when asked why he fights. "Why do people play chess?"

Mixed martial arts, or MMA, first became a form of entertainment in Brazil and Japan. It debuted in the U.S. in 1993. Today, the sport puts on about two dozen major events a year here, mostly on pay-per-view television. Although there are other promotional bodies and feeder leagues, the UFC is the largest and most established: It hosts fights in largest venues, purses and bonuses for major fights can surpass $2 million, and is now sanctioned in 37 of the 45 states with athletic commissions.

In 2000, boxing veterans and Las Vegas casino owners Lorenzo and Frank Fertita bought a stake in the UFC and won state approval. (This year, three MMA events organized by a UFC rival were shown on CBS.)

Mr. Couture first heard of MMA in 1996 when one of the wrestlers he coached at Oregon State gave him a tape of an event -- and one of the fighters was a former Oklahoma State wrestling teammate. "I was immediately intrigued," Mr. Couture says. After serving as an alternate on the 1996 Olympic Greco-Roman wrestling team, he applied to the UFC, but was turned down, he says, because the organization felt it had too many wrestlers and not enough martial artists.

The following year, though, he was asked to take the place of an injured competitor in a UFC tournament. Nine days later, at age 34, he won his first fight, beating Finnish boxer and pro wrestler Tony Halme in 57 seconds by forcing him into submission, then went on to win the final in just over three minutes. By December, "The Natural" had earned the UFC's heavyweight title.

"They threw him to the lions," says Stephen Quadros, a fighting analyst and commentator who considers Mr. Couture the greatest champion in UFC history. "He was older, he wasn't flashy, he wasn't a smack-talker, but he proved what he proved."

Mr. Couture has fought 18 times for UFC and has shown a remarkable ability to add new styles and tricks to his repertoire. His 13-5 record doesn't quite tell the story: His first UFC loss came at the hands of a fighter who later tested positive for steroids; another came on a cut; and two more came at the hands of Chuck Liddell, the great light heavyweight with whom Mr. Couture fought a thrilling trilogy that helped cement the sport's growing popularity.

After retiring for a time in 2006, Mr. Couture returned to the ring in March 2007 to face heavyweight champion Tim Sylvia, a boring but effective jab artist who was 40 pounds heavier, nearly a foot taller and more than a decade younger. In an electric moment, Mr. Couture, a heavy underdog, came straight after Mr. Sylvia in the first 10 seconds and knocked him nearly clean out with a straight right hand. Mr. Couture spent the rest of the fight backing the visibly shocked Mr. Sylvia into the cage with clinches and grinding him into the mat. As the clock counted down the last 10 seconds, a delirious crowd of 19,000 counted along; the Natural was again heavyweight champion. In his most recent fight, in August 2007, he knocked out Gabriel Gonzaga, a monstrous young Brazilian submission specialist, in the third round. "I try to break them," Mr. Couture says of his opponents. "Break their will. Push them further than they're willing to go."

On the eve of his fight with Mr. Lesnar, mixed martial arts seems close to doing something rare for a developing sport -- claiming a mainstream audience. UFC has expanded into Europe and, according to Mr. Fertita, the chief executive, is looking at Dubai and the Philippines. It has attracted some corporate sponsors and is hosting fights at major venues where up to 20,000 fans pay as much as $1,000 a ticket. UFC's main competitor, Elite XC, which had broadcast deals with CBS and Showtime, filed for bankruptcy-court protection last month. Sen. McCain recently told a reporter the sport had "grown up."

Nevertheless, the sport's fans skew young and many people can't distinguish it from scripted events like pro wrestling. States like New York and Massachusetts haven't sanctioned it and critics say there are too many amateurish fighters. There's some talk about whether Mr. Lesnar, a former high-profile pro wrestler with only three mixed martial arts fights, belongs in the limelight so soon.

Mr. Lesnar, a former national wrestling champion at the University of Minnesota, hits harder than arguably anyone in the game, says Mr. Quadros, who predicts Mr. Lesnar will win by second-round knockout. "But if Lesnar can't beat him in the first two rounds, Randy is going to stretch him out."

Sean Van Patten of Las Vegas Sports Consultants, who helps set betting lines for casinos, says he opened with Mr. Couture as the favorite, but as of late Thursday had Mr. Lesner as a very slight favorite. The longer it lasts, he says, the more chance Mr. Couture has to catch Mr.

Lesnar in a mistake. To win, Mr. Couture says he will have to hold his ground, counter Mr.

Lesnar's wrestling with better wrestling and, as he put it, "hit him every chance I get."

"Brock's game plan is going to be to use his size to overwhelm me, take me down," Mr. Couture says. "I'm an aggressive person by nature, and in my fighting style, so I'm going to go out and impose my will."