A sellout crowd of 18,130 turned up at Capital Centre last night. Many left before the third period.

The New York Islanders extended their unbeaten streak to six games by humiliating the Washington Capitals, 5-1. The Capitals have won only one of their last nine.

This was Washington's most one-sided defeat this season. The worst previous loss, 8-5 to Edmonton, came during the Centre's other sellout.

Mike Bossy, who has 24 goals in 20 games, and Mats Hallin, drafted by Washington in 1978 but never signed, each scored two goals for the Islanders.

Goaltender Kelly Hrudey had 31 saves in his first start against Washington, but lost a bid for his first NHL shutout when Scott Stevens scored in the third period.

It hardly seemed fair, but after the noncontest the Capitals assigned to Binghampton defenseman Dave Shand, who has played only one game in a month.. His only connection with last night's game was to man the phones behind the bench.

Bob Mason, who played so well in Pittsburgh Wednesday, was rewarded with a second straight start in the Capitals' nets. He lasted 15:57, yielding three goals in eight shots before Pat Riggin relieved.

"We wanted to find out if Bob Mason is close to playing in the NHL," said Washington Coach Bryan Murray. "In Pittsburgh, the way he played in the first period was the difference between losing and getting a tie out of it.

"But tonight, the first shot beats him and he struggles. I had to get him out to protect him as much as anything."

Actually, Mason stopped the first shot that came his way. That was a point-blank save on Greg Gilbert following a giveaway by Rod Langway, whose subpar play continues.

Bossy opened the scoring on the Islanders' second shot, a short-side drive from the left-wing circle. He unloaded it an instant before his old junior teammate at Laval, Glen Currie, flattened him.

"I got the puck and I thought about passing to the point, because there were two Washington guys there," Bossy said. "When I pulled it in toward me, they sort of tailed off, so I was able to get a shot off.

"I feel I'm more an opportunist than anything else. Tonight I got three chances in their end and I got two goals and an assist out of it."

Hallin, who had watched the last four games from the stands, beat Mason from the same spot less than three minutes later. Mike Gartner, whose 17-game scoring streak ended, and Gary Sampson had missed chances at the other end before Stefan Persson got the puck out and fed Hallin.

After Hrudey stopped Alan Haworth's close-up shot, Hallin made it 3-0 at 13:47 of the first period. Mason did a good job to block Hallin's deflection of Bob Bourne's shot, but he was down and deserted by teammates when Hallin took the rebound and flipped a backhander over him.

"I ask before every game if I'm going to play and today I'm happy the answer is yes," Hallin said. "I'm lucky to get two goals. Maybe that's the break I need to get going, get some confidence."

Hallin said the Capitals didn't show much interest in him. "They had one scout in Sweden who said I was drafted," he said, "but I never got an offer from them." He signed with the Islanders in 1981.

New York scored twice in the second period. When Doug Jarvis could not control the puck at the New York blueline, Bossy raced past him down the right side and, as Riggin came out, sent a well-placed shot past him into the far corner of the net.

After Haworth put the puck over the net on a setup by Stevens, Bossy and Bryan Trottier started a two-on-one break, with Trottier converting.

Hrudey foiled Gaetan Duchesne's breakaway late in the second period, but, early in the third, Darren Veitch checked the puck from Pat Flatley, then Stevens skated between most of the Islanders and beat Hrudey from the left-wing circle.

Bossy has 22 goals in 20 regular-season games at Capital Centre, at least one in his last seven appearances. He also had two in the 1982 All-Star Game at the Centre.

"I don't know what it is about this building, but I seem to be able to put the puck in the nets here," he said.

In other games, Tim Kerr got the tie breaker in Buffalo as the Philadelphia Flyers beat the Sabres, 4-2; in Detroit, the Red Wings beat the Maple Leafs, 6-5, on Kelly Kisio's two goals; in Bloomington, Minn., the North Stars needed one shot in overtime -- Craig Hartsburg's at 3:35 -- to beat the New Jersey Devils, 5-4; in Calgary, Doug Gilmour got the game-winner in the St. Louis Blues' 3-1 victory over the Flames; and in Vancouver, Dale Hawerchuk of the Winnipeg Jets got two goals and two assists in a 5-1 victory over the Canucks.