Never have 36 minutes of fisticuffs meant so much to the careers of two men as the 12 rounds of mayhem scheduled for tonight mean to Muhammed Ali and Joe Frazier. Because, for one of the pugilists in tonight's Madison Square Garden match--the one who doesn't come away with the winner's laurel--the end of the boxing road will be at hand.

Ever since their "fight of the century" encounter three years ago, Frazier and Ali have experienced some pretty lean times. For the flamboyant Ali, there has been the frustration of waiting for a rematch with Smokin' Joe, and more recently, the shattering of his reputation (and jaw) at the hands of Ken Norton. And since the last go-around, Frazier has undergone the drawn-out recuperation from the facial pummeling Ali gave him and, most recently, the humiliating de-regalization by George Foreman as heavyweight king.

Both fighters are now confronted with the slide into the twilight of their careers. Past 30 years old, the watershed of many a fighting career, neither can afford a further tarnishing of the record to hasten the skid from the top. And with an awesome new champ holding court today as king of the heavyweights, tonight's Ali-Frazier is a battle of the "once-weres" not the "here-and-nows."

For Ali, perhaps the most versatile heavyweight who ever pulled on a pair of padded gloves, the passage into the dusk of his fighting career must be a particularly bitter one. Robbed of three productive years in his fighting prime for embracing unpopular political beliefs, Ali, since re-emerging into the harsh glare of center ring, is no longer the lightning quick and powerful fighter who was banished for prophetically asserting that as an American, he "had no quarrel with them Viet Cong." As penalty for this vision, which he could claim long before it came into vogue, Ali was stripped, not only of his livelihood and title, but of his incomparable skills as well.

For Frazier, always a poor-relation titlist for most Americans after the ostracism of Ali, tonight's bout is a chance to silence once and for all those who have contended that he is not a blue-chip heavyweight. Despite his unanimous decision over Ali three years ago, Frazier has still to silence the ghost of Ali taunting him over his shoulder.

If Ali were still in his prime, or even if he had not been driven from the professional ranks for three years, the outcome of this fight would be no more in doubt than those sparring sessions against the Henry Coopers, Karl Mildenbergers, and Zora Foleys previously. But Ali is no longer in his prime; it has been ten years since he first wrested the title from Sonny Liston, and the one-time champ is now 32 years old.

The first Ali-Frazier fight was everything it was billed to be. This one is not likely to measure up as well. Since the last contest, both fighters have shown distinct signs of slippage. Ali has lost the zip of his youth, if we are to judge from his two lackluster performances against Norton, and Frazier was pathetically outgunned against Foreman.

Ali no longer seems able to "float like a butterfly, sting like a bee," and Frazier has passed that point in his career when he reigned as the most intimidating boxer to step into the ring since Liston.

Tonight's fight will not determine the heavyweight champion of the world. George Foreman has staked his claim quite convincingly to that role. Rather, tonight's battle must be viewed as a showdown between two fighters, each champions in their own era, seeking to brake the slide from the top. It is a fight for survival.