Once again using the third quarter as their springboard to excellence, the Boston Celtics took a 2-0 lead in the 1986 NBA championship series tonight by defeating the Houston Rockets, 117-95, at Boston Garden.

As in their previous playoff series against Chicago, Atlanta and Milwaukee, the Celtics emerged from the locker room after intermission and took away any semblance of competition in the second half tonight. As in other games, their dominance was on both offense and defense.

In their 13-1 postseason run, the Celtics have outscored their opponents by a total of 88 points in the third quarter, an average of more than six points per outing. They more than doubled that average tonight, outscoring Houston by 34-19 during the period. The Celtics hit 14 of 27 field goal attempts and limited the Rockets to eight-for-20 shooting in the quarter.

Leading the way for the winners was forward Larry Bird, who was named the league's most valuable player Wednesday for the third straight year. Tonight, Bird transformed the building into an individual theater and mesmerized most of the crowd of 14,890, the 269th consecutive sellout here.

Bird scored nine of his 31 points, had four of his eight rebounds and passed off for three of his seven assists during the decisive third period. During a span of a little more than a minute -- between the 7:59 and 6:52 marks of the quarter -- Bird fed Robert Parish for a layup, hit his third three-point field goal of the night, then added a 21-foot jump shot for good measure.

When he left the floor with 3:57 to play in the game, the seven-year veteran received a standing ovation, but he barely had time to settle into his chair. After two quick baskets by Houston forward Rodney McCray, Boston Coach K.C. Jones hustled Bird back in. He had sat for all of 58 seconds.

In the remainder of the quarter, he got an assist and a pair of baskets, the last a leaning layup with 10 seconds to play that drew another standing ovation.

Those points and most of what Bird accomplished tonight on offense were spawned from a one-on-one, isolated set, the forward on one side of the floor with his teammates spread about on the other. If McCray tried to play him alone, the taller Bird would get his shot off after a number of fakes. If Houston chose to double-team, he would merely pass off to a cutter for an easy basket.

"I don't think they can handle it," he said of the isolation. "I think it's to our advantage to use it; I think it's our best play."

It's certainly one that has frustrated McCray and the Rockets more than a little.

"If we're going to win this series, we have to do a better job on that play, there's no question," said McCray. "I have a lot to think about between now and Sunday when Game 3 will be played at the Summit in Houston ."

Rockets Coach Bill Fitch will spend much of the interim trying to decide whether to change a starting lineup that has played in fits and starts through the opening two games of the series.

In Game 1, center/forward Ralph Sampson suffered through a miserable game, a result of foul troubles that led to a two-point, one-for-13 effort.

Tonight, Sampson scored 18 points and his fellow Twin Tower, Akeem Olajuwon, had a team-high 21. The stronger dual effort was offset, however, by a seven-for-19 night by starting guards Robert Reid and Lewis Lloyd. The duo finished the night with a total of 14 points, 19 fewer than Danny Ainge and Dennis Johnson, the Boston starters.

"We may shake some things up just to change the chemistry," said Fitch. "Our starting five just hasn't played well, in the third quarter or at the start of the game."

After Lloyd put the Rockets on top, 2-0, with a layup, the Celtics scored eight straight points and 11 of the next 13. Kevin McHale (25 points) had the first six, but the run was capped by Bird's first three-pointer of the night.

The Rockets rallied to close to 31-30 after the opening quarter. Sampson scored 10 of those points for Houston but early in the second period he suffered a gash beneath his left eye after jostling for a loose ball with Parish.

Sampson retired to the locker room, where he received five stitches. He returned just in time to catch the start of Bird's show. By intermission, the Celtics were ahead, 60-50, and Bird had 20 points. That was but a prelude for the third quarter.

"K.C. has a philosophy that when a man has a hot hand, you continue to go to it," said Boston assistant coach Jimmy Rodgers. "And when a fire is going like that, you throw gasoline on it to make it stronger and keep it going."

Although the next three games are scheduled to be played in Houston, what once looked like a favorable setup for the Rockets suddenly looks as if it won't make much difference. Had the Rockets achieved at least a split here, they would have been in commanding position. Now the team has almost been left begging for any kind of victory.

"Now we get to go to our own place, eat our own food and sleep in our own beds," said Sampson. "I don't care what they do or how they play. I care about the Houston Rockets -- and we have to play tougher, rebound, block shots and run more. If we can do that, we'll be in good shape."