Luis Clemente Tiant returned to the big leagues tonight.

At age 40, bellied, balding and barely two days after the Pittsburgh Pirates rescued him from their Portland farm club, Tiant embarked upon his 18th season in the major leagues. He performed well, even though he did not win.

The record will show he surrendered six hits and five runs in a 7-2 loss to the Montreal Expos at Three Rivers Stadium. But Tiant and the 11,735 fans know his performance was better than the numbers.

Tiant held Montreal scoreless on one hit through the first five innings of his second attempt at a major-league comeback. Andre Dawson, the Expos' All-Star center fielder, was hitless three times against Tiant, although he contributed to the Expos' victory with a grand-slam home run on the first pitch after Tiant left the game in the seventh inning.

The Expos got to Tiant for an unearned run in the sixth before loading the bases in the seventh. Teammate Dale Berra's errant relay on a potential double-play ball hastened Tiant's departure.

After one-out singles by Larry Parrish and Mike Phillips, winning pitcher Steve Rogers attempted to bunt the runners ahead. Instead, he grounded sharply to third baseman Bill Madlock, who turned and threw to Berra, covering at third. Berra's relay to first should have beaten Rogers. It didn't.

Tiant then yielded a run-scoring double to Tim Raines and walked Rodney Scott on a 3-2 pitch to load the bases. That brought Pirate Manager Chuck Tanner to his feet.

Tanner summoned reliever Victor Cruz, who promptly ruined Tiant's Pirate debut, giving up the home run to Dawson.

Cruz then pitched another home-run ball to the next batter, Gart Carter. Immediately after the game, the Pirates optioned Cruz to their Portland farm team -- the team Tiant just left.

"I know I pitched good enough to win," Tiant said afterward, waving the cigar that accompanies him even to the shower. "It's happened to me before."

At 40, Tiant winces at the recurring digs about his age and memories of the silent telephone after he pitched a no-hitter and two one-hitters at Portland earlier this season.

"I see all those young guys there. They are supposed to be prospects," he said. "I know I can pitch better than 90 percent of them. But everyone looks at my age. They wonder if I am really 40; 40, 50, 100, what's the difference? No one asked how old is Pete Rose."

Any questions about the soundness of Tiant's arm should have been settled at Portland, where he posted a 13-7 record and a 3.87 ERA in a hitters' league.

"My arm is good as ever," he said. "I'm not going to retire just because somebody says I'm too old. They don't pay me for my age. They pay me to pitch. I know what I have to do. I've been through this before -- I know what it is to come back. I'm not impressed by the situation. If I can't win with my best stuff, there's nothing else I can do. But I know I can pitch in the big leagues."

For a while the Pirates were not convinced. As their pitching staff disintegrated, with John Candelaria and then Don Robinson encountering arm trouble, the Pirates left Tiant to wait and wonder in the minor leagues.

On Tuesday, General Manager Pete Peterson, sensing the need for a gate attraction and perhaps a win or two, paid Portland the piddling sum of $25,000 and assumed Tiant's $100,000-per-year contract.

"I said it in spring training, 'I'm going to come back,' " Tiant said softly. Perhaps life really does begin at 40.