Danny Belisle threw in the towel last night. The Washington Capitals' coach heaved a blue towel in disgust as his skidding hockey team messed up a six-on-four advantage in the closing minutes of a 6-4 loss to Buffalo.

As usual when a big crowd comes to Capital Centre, there were giveaways. First the Capitals handed ski hats to early arrivals in the crowd of 17,196, second largest of the season. Then they blew a 4-2 lead and gave the game to the Sabres, who came here with a four-game losing streak.

"Mental errors cost us goals, just minor-league mistakes" Belisle said. "There were too many three-on-twos tonight. That's against our system, our philosophy."

The Sabres outshot the tiring Capitals, 17-7, in the third period and shattered a 4-4 deadlock on a pair of goals by Rene Robert, who has been in such a tailspin that they raised his season total to only 14.

With 5:04 to play, Robert skated past defenseman Rick Green, who offered only token resistance, and attempted a shot that struck goalie Gary Inness' pad and caromed high in the air. Robert made a midair deflection to score on the rebound.

"I made a good move to get around Green, but the goalie poke-checked me and the puck bounced in the air and hit my stick and went in the net," Robert said. "That's what you call luck."

From Belisle's uncomfortable viewpoint in front of disapproving fans, more than luck was involved.

"You can't get three guys caught when you've got a tie game," Belisle said, "and on that goal we had three guys caught. (Greg) Polis was caught in the corner and (Dennis) Maruk and (Tom) Rowe were in deep."

The clock had wound down to 2:51 when Andre Savard took the puck away from Washington's Robert Picard, who suffered through his worst game in two months, and fed Robert to complete a two-on-one break against deserted Leif Svensson.

When Buffalo's Lee Fogolin was penalized for holding with 1:46 remaining, Belisle pulled Inness for a sixth skater. His frustrations at the inefficiency of what followed were matched only by those of Savard, who twice missed the empty net. Savard shattered his stick on the boards at game's end and received a misconduct penalty and automatic $50 fine.

The loss brought the Capitals to the three-quarters mark of the season with an 18-33-9 record, 13 points away from a playoff berth, and the immediate future offers little comfort, since the Montreal Canadiens are 7:30 guests tonight before the Capitals leave on a West Coast trip.

Although Buffalo's Danny Gare opened the scoring, last night seemed to be a Capital evening, since Polis and Bob Sirois scored before the first period ended for a 2-1 lead. Then, after Gil Perreault had briefly tied it, Rowe and Sirois connected 59 seconds apart for a 4-2 Washington lead.

Polis' goal created about the only glee evident from the crowd, which exhibited a minimum of enthusiasm despite the closeness of the score. Polis and Maruk exchanged some sharp passes before the final setup left Polis in position to deke goalie Bob Sauve and lift the puck over him.

Rowe's 21st goal was also aided by Maruk's passing, as the 5-foot-8 center cut past 6-4 Gerry Korab of the Sabres and dropped a pass that left Rowe open.

Two accurate shots boosted Sirois to the 25-goal mark for the first time. Sirois, who collected 24 a year ago, is the team leader in that department. ment.

Picard was victimized by Ric Seiling to begin the Sabres' comeback. Seiling skated right past the usually reliable defenseman and lifted a shot over Inness' left shoulder for his 15th goal.

Richard Martin cut behind Robert's perfect pass for another breakaway score while each team was a man short. It was 4-4 and, for the Capitals, everything else was downhill.

Washington is enmeshed in a five-game winless streak and has suffered through a 1-6-1 February, losing all four home games, after that memorable 9-5-1 step forward in January. The Capitals are ready for March, although it seems too late for a playmarch.

Paul Mulvey, the 6-foot-4 left wing who showed much promise in October but was eventually sent to Hershey, came back to the Washington Capitals last night. Assigned to the American Hockey League club in his place was left wing Gary Rissling.

Mulvey, who scored 10 goals in 24 games for Hershey, netted two in his first three NHL contests. After 27 scoreless contests in which his playing time was reduced to almost nothing, he was demoted Dec. 26.

Rissling also started fast in the NFL. After he was called up Jan. 2, he recorded three goals and three assists in 10 games, building a plusseven rating. In his last six games, however, he was scoreless while suffering through a minus-10 stretch. He was leading the Capitals in penalties, serving 123 minutes in only 22 games.