The Boston Bruins spotted the New York Islanders a two-period head start tonight in the first of their best-of-seven Stanley Cup semifinal series. The Islanders used it to build a 4-0 lead and withstand a late Boston rally for a 5-2 victory.

The Islanders dominated the first two periods, getting goals from Brent Sutter, Thomas Jonsson, Mike Bossy and Duane Sutter. The Bruins managed 22 shots but few good ones. They appeared to be suffering a physical and emotional letdown from the seventh-game overtime victory over Buffalo Sunday in the quarterfinal round.

In the third period the Bruins took control, with Barry Pederson and Ray Bourque scoring 17 seconds apart. But further Boston pressure proved futile, and Bob Bourne scored the New York's fifth goal on a break-in late in the game.

The line of center Brent Sutter and wingers Bourne and Duane Sutter had three goals, continuing the red-hot pace it set in the quarterfinal series against the Rangers.

Islander Coach Al Arbour said his team, three-time defending champions, put together a strong game in all areas.

"We wanted to get the jump on them because there was so much talk of them being tired," said Arbour. "But in the third period they sure didn't look tired. They skated at us very well but we were able to tighten it up again."

Bruins Coach Gerry Cheevers said the fatigue factor was not significant.

"We just got outplayed," said Cheevers. "It happens. They dominated on the boards and at the points in our end and their end."

Three minutes into the game, Brent Sutter put the Islanders ahead, 1-0. Bourne took the puck from Pederson and dropped it to Sutter in the left circle. Sutter's shot beat goalie Pete Peeters on the inside angle, Duane Sutter providing the screen.

Defenseman Jonsson made it 2-0 with an unassisted goal midway through the second period. Jonsson took the puck at left point, drove the middle by getting past the Boston winger, and found himself unguarded halfway up the slot. His wrist shot was partially gloved by Peeters but toppled into the goal.

Bossy score the third goal for New York on a power play, connecting on a drive from the left circle. Two nifty cross-ice passes by Jonsson and Denis Potvin drew the Boston defense out of position and set up the clear shot.

Bourne, who was the best player on the ice, set up the fourth New York goal. Bourne rushed the left side and made a clever move deep in the Boston zone to cut inside on defenseman Mike O'Connell. Moving in on the net, Bourne laid a short pass across the slot to Duane Sutter, who had only to tip in a two-footer.

With all this scoring, Peeters turned away several other penetrations by the Islanders. But the Bruins mounted almost no pressure on the Islander goal, despite having two power plays.

"I couldn't have been happier with the second period," said Arbour.

The third period proved to be different, as the Bruins attacked furiously from the first faceoff. Cheevers' use of backup defenseman Larry Melnyk and Marty Howe to spell his depleted defensive unit may have made the difference.

Pederson broke through with a backhander in front, taking a feed out of the corner from Mike Gillis, who had gotten to a loose puck three feet ahead of the closest Islander.

The same shift was skating for Boston when Bourque took a lead pass from Pederson and broke past the Islander defense at the Islander blueline. Bourque got Billy Smith to commit for a forehand shot before chipping in a soft backhander.

The Bruins bottled up the Islanders for much of the rest of the game, to no avail. Bourne blocked a shot by Brad Park late in the game, broke out alone, and beat Peeters.

The third period was marked by considerably more body checking on the part of the Bruins, a possible indication of the course of Game 2 Wednesday night.