Shortstop Cal Ripken hit a 10th-inning home run and reliever Nate Snell got his first victory of the season as the Baltimore Orioles beat the Seattle Mariners, 5-4, tonight before 16,230 at the Kingdome.

Ripken's homer, his sixth of the season, brought the Orioles back after they had wasted a 4-0 lead, the final blow coming on Seattle designated hitter Gorman Thomas' two-run homer off reliever Don Aase in the ninth.

The victory is the Orioles' fourth straight and 12th in 14 games. At 24-16, they are within 2 1/2 games of the first-place Boston Red Sox in the American League East.

To get this one, the Orioles had to use four pitchers and outhit the Mariners, 13-6. They also had to get 1 2/3 innings from Snell, who has been used strictly as a mop-up man since being recalled from the minor leagues April 26.

As the Orioles continue to win, the only negative is that their pitchers aren't finishing games. They have had only one complete game since April 29, a stretch of 21 games, and the fatigue appeared to show tonight on Aase, who made his fourth appearance in five games. Aase, who has been extremely effective, didn't come through this time.

He struck out first baseman Alvin Davis but got behind Thomas by a 2-1 count and put a fastball across the middle of the plate. Thomas hit it into the right field seats, 350 feet from home plate, for his ninth homer.

Seattle third baseman Jim Presley followed with a single to center and Weaver brought in Snell, who got the Orioles out of the jam in the ninth and finished the game.

Aase had not allowed a run in 10 straight appearances and had seven saves and a victory in his last nine.

"Really, I felt better tonight than last night," he said. "I just got the pitch up in the strike zone, and he killed it."

Orioles Manager Earl Weaver was not about to criticize his bullpen ace, especially after Ripken's homer gave the Orioles the victory anyway.

"You know you're going good when your big gun Aase gives it up and you still win," Weaver said. "He had to do it sooner or later. It wasn't going to last forever. . . . Ripken hits one when you least expect it. when you're in a slump hitting .241 and get three game-winning homers in five days, you're a pretty good hitter."

Thomas' homer wasted 7 1/3 innings of three-hit pitching by Orioles starter Mike Flanagan, who has not won since April 17.

"I feel bad for Flanny," said Weaver, "but he won't need a lot of luck if he keeps pitching like this."

Flanagan faced only 12 batters the first four innings before yielding two runs in the fifth. And despite Flanagan's effectiveness, Weaver went for Rich Bordi when Mariners catcher Steve Yeager got a one-out single in the eighth.

The Orioles had a 4-0 lead after they sent nine men to the plate and scored four times in the fourth inning, knocking out 24-year-old Seattle starter Billy Swift. In the inning, they got a leadoff double from Larry Sheets, a run-scoring single from Mike Young, an RBI fielder's choice from Lee Lacy and a two-run single from Eddie Murray with the bases loaded.

Reliever Edwin Nunez got Ripken on a ground ball to Davis to end the inning after Murray's hit.

Flanagan, meanwhile, was almost perfect the first four innings. The only baserunner against him was Dave Henderson, who drew a one-out walk in the third.

Yeager then hit into an inning-ending double play, which allowed Flanagan to face only 12 batters the first four innings. Of the 12 outs, eight were on ground outs, two on strikeouts.

Flanagan's string was broken in the bottom of the fifth when he gave Thomas a one-out walk and Presley grounded a single to left for the Mariners' first hit.

Center fielder Al Cowens then doubled to right-center, scoring Thomas to cut the Orioles' advantage to 4-1. Henderson then scored Presley with a ground out to Wiggins at second.

But Flanagan struck out Yeager to end the inning and keep the Orioles' lead at 4-2. He sailed through the sixth and seventh before departing in the eighth.

"That's the best I've pitched this year over all," said Flanagan. "No, I wasn't mad about coming out. What chance have I got? The man walks out of the dugout and waves the guy in. . . . He's the manager. I'm the pitcher."