Washington Capitals, well rested and often boastful of their superb conditioning and consequent third-period capabilities, carried a 2-1 lead into the last 20 minutes tonight against a Los Angeles team that was playing its fourth game in five nights.

Instead of walloping the weary, the Capitals came undone. Rookie Greg Terrion scored two goals and another youngster, Larry Murphy, put the clincher into an open net as the Kings outshot Washington, 16-5, and rallied for a 4-2 victory.

"I wish I knew," Washington Coach Gary Green said in reply to the obvious. "We did not hit them in the third period. We became mediocre in our end zone. We were playing a skating, defending-the-pass game rather than playing the body.

"It showed us something out there tonight.It gave us something to think long and hard about tonight."

For added thought, center Guy Charron will be X-rayed Thursday in Denver -- where the Capitals play the Rockies Thursday night -- for what is at best a painful bone bruise in the lower back. Charron tumbled over goaltender Mario Lessard and landed on his back after firing a second period shot that struck a goal post.

"It felt like a jab in my back; I felt it right away," Charron said. "I tried to stay in, but I wasn't mobile enough to do some things, like fighting in the corner, so I thought I'd better stay out.

The Capitals spent two practice sessions here working on their mediocre power play and it paid off with the only goal of the first period, Bob Kelly connecting on a rebound at 16:31 with the Kings' Dan Bonar off for interference.

Dennis Maruk utilized Jean Pronovost's pick to come around from behind the net and get off an unimpeded shot at Mario Lessard. The goalie managed to block it, but Kelly, playing his 750th NHL game, lifted the rebound over him. Kelly had also scored Washington's last goal, the one that created the 3-3 tie in St. Louis on Saturday.

Washington, fresh after three days in the California sun, had a 15-4 edge in first-period shots on the Kings' who were playing their fourth game in five nights.

Los Angeles was so badly out-played that Coach Bob Berry changed tactics midway through the period and tried some different mathchups. Of particular concern was the Capitals' line of Alan Hangsleben, Guy Charron and Bengt Gustafsson, which pressured Lessard every time it came on the ice.

The Kings' potent Triple Crown line of Charlie Simmer, Marcel

Dionne and Dave Taylor, 1-2-3 in NHL scoring, was well defensed by Maruk, Kelly and Pronovost, so Berry tried them against Washington's fourth line of Rolf Edberg, Wes Jarvis and Dennis Ververgaert. Edberg was playing his first game since Oct. 19, with defenseman Yvon Labre sitting out, because Washington Coach Gary Green wanted added forechecking pressure on the Kings' young defense.

The game turned around in the second period, with the Kings rolling up a 14-6 margin in shots, but the Capitals managed to hang onto a 2-1 lead.

With Washington's Rick Green off for holding Simmer, the Kings' potent power play clicked for the 15th time this season. Taylor, behind the goal line at the right of Washington netminder Mike Palmateer, reached far out and swept the puck behind the little goalie at 1:59.

It took the Capitals only 28 seconds to regain the lead. Kelly, in the right-wing corner, fired a centering pass to Pronovost in the slot. Pronovost fired and scored just as he was flattened by defenseman Jay Wells. It was Pronovost's first goal as a Capital, the 369th of his NHL career.

Washington appeared to score again at 9:50 as Edberg lifted a shot over Lessard's left shoulder, but referee Bruce Hood ruled Washington's Pat Ribble had struck the puck with his stick above his shoulder before it skidded to Edberg.

While Washington rookie Darren Veitch served a holding penalty late in the period. Guy Charron intercepted a pass at the Los Angeles blueline and skated in to hit the goal post with a bid for a shorthanded goal.As Charron shot, he was knocked off balance, crashing against the boards behind the net. He rose slowly and skated off, but he did not go to the dressing room.

Palmateer was outstanding in the period, unloading several long passes that helped to ease Los Angeles pressure and making several outstanding saves. His best came as he was handcuffed by a Terrion shot, but managed to bat the puck back over his head and behind the net.

The Capitals had a couple of good chances to increase their lead, but Lessard foiled Gustafsson, on a deflection of Jarvis' pass into the slot, and stopped a drive by Ververgaert, set up in front by Gustafsson.

Palmateer made some excellent stops and two quick ones on Simmer who was unguarded in front of the net left Simmer banging his stick on the ice in disgust.