It is not a coveted assignment to play the Montreal Canadiens after they have lost a game. The Capitals caught the Stanley Cup champions on the rebound last night and suffered a 3-0 defeat despite an excellent defensive effort.

The Canadiens scored in each period at Capital Centre and their 6-foot-4 all-star goaltender, Ken Dryden, stopped all 30 Washington shots. It was Dryden's sixth shutout of the season, No. 2 against the Capitals.

Dryden has blanked Washington five times over three seasons, so he is something of an expert on their moves and countermoves.

"They are a remarkably different team from last year," Dryden said. "To improve so much in a year's time with the same personnel is a real accomplishment. A lot of players are improving here."

The Canadiens have average 37 shots a game and last night they managed only 23. But that total was not a disappointment.

"We had to play a pretty good defensive game," said coach Scotty Bowman. "We've been getting away from it. We've been scoring a lot of goals and getting carried away. A defensive game will win the championship."

The Canadiens were beaten, 7-3, in Boston Monday night. Last week they dropped a 7-2 verdict in St. Louis, then jumped all over Los Angeles, 6-0. So last night's resurgence was predictable.

"We've never lost two in a row," Bowman said. "We're always able to come back. If we grind it out, the edge will go to us, unless we do something foolish."

The Canadiens did nothing foolish. They swarmed over the Capitals in prime scoring territory and it was a rare Washington player who found himself open in the slot. One such was Bryan Watson, late in the first period, but Dryden was in the way.

"I was surprised it opened up so much," Watson said. "I got it where I wanted it, but he covers so much net."

Mike Marson had another glorious opportunity early in the third period, as he took a perfect pass from Bill Riley and tried to beat Dryden with a backhander. The big goalie flopped, covering the entire six-foot area from post to post, and Marson was unable to lift it over him.

At the other end, Bernie Wolfe was making the more spectacular saves, as each Washington error was quickly followed by a sudden thrust toward the net.

Right wing Guy Lafleur, the NHL's leading scorer, twice slipped down the off wing to feed Jacques Lemaire in close. The first time, Lemaire shot wide. The second time, his rising shot struck Wolfe in the left shoulder and caromed high into the net, just inside the goal post. The goal, Lemaire's 21st, came at 8:40 of the first period.

Montreal managed only five shots in the second period, but one vicious blast by an unchallenged Yvon Lambert struck Wolfe in the chest and, as the goalie was spun around, the puck dropped behind him. That was Lambert's 13th goal and, although 33 1/2 minutes remained, it was apparent the Capitals' task was a most difficult one.

It did not become impossible until 2:04 left, when Doug Risebrough converted Mario Tremblay's pass from behind the Washington goal lie.It was No. 12 for Rissbrough.

The shutout was only the third of the season against the Capitals and first in 35 games, since a 7-0 drubbing in Atlanta Nov. 3. Still, they have scored only 17 goals in their last 10 games and that makes winning difficult, even though their 26-goal yield over the same stretch is the lowest in the NHL.

Bowman found no fault with the defensive tendencies, however, particularly since he has adopted such tactics for himself.

"They have tightened up a lot," Bowman said of the Capitals. "They are better off to play tight, even though it is tough to encourage the team and the fans. Their defense is coming along and they have good size in (Gordie) Lane and (Rick) Green. They've got hustlers up front. It's a matter of picking up a player here and there."

Montreal is loaded with surplus talent, but Bowman was making no offers on behalf of his boss, Sam Pollock.

Lafluer, a standout as unusual despite a sore wrist, commented, "The Caps have really improved since last year. They're tougher to beat. They have improved the defense, but they have to be more aggressive in front of the net."

Two of the NHL's better fighters. Lane and the Canadiens' Pierre Bouchard, squared off in the first period after an exchange of high sticks. Lane wrestled his taller, heavier opponent to the ice. A flareup between Lambert and Watson after Lambert's goal ended in similar fashion, the two rolling on the ice.

Washington's Ace Bailey received a 10-stitch cut over his right eye when he was struck by Green's slap shot in the first period. He returned to action, and later commented, "How many times do I have to get hit before I'm smart enough to wear a helmet?"

The Capitals return to action tonight at 8 in Buffalo, but Bailey won't be trying out any headgear. The swelling promises to be so big that a helmet won't fit over it.

Washington winger Mike Lampman underwent a 4 1/2 hour operation yesterday for slipped vertebrae and Dr. P.M. Palumbo Jr. pronounced it a success. Lampman is recuperating at Arlington Hospital.