As President and Mrs. Reagan solemnly chanted the Christmas refrain "peace on earth" last night, the word "peace" seemed to take on a special meaning.

The first couple, attending the annual taping of NBC's "Christmas in Washington" special at the National Building Museum, exchanged moist and smiling looks as they sang. The invited audience of Washington political and media powers crackled not only with the spirit of the evening, but also with the spirit of the week that saw the United States and the Soviet Union sign a treaty.

Before the president joined the caroling on stage he delivered his own holiday message. Retelling the biblical story of the first Christmas, he said, "We must always remember that message first sung by angels almost 2,000 years ago: Peace on earth, goodwill toward men. Twenty centuries later their song remains humanity's prayer."

Though Reagan didn't directly mention last week's accomplishments, others certainly linked the evening's euphoria with the president's success. Country singer Barbara Mandrell, the show's emcee, said, "The hope of the summit is still in the air. This year there is a greater spirit of peace on earth."

By now the hourlong taping and lavish reception are Washington institutions. Decorations of Christmas trees, antique dolls, a giant sled with four reindeer, and strings of bows and lights filled almost every foot of the mammoth Building Museum. The tables groaned under sweets, drinks and hors d'oeuvres, and were adorned with giant gingerbread houses, toy soldiers and Santas.

The guests, who included Defense Secretary Frank Carlucci, Education Secretary William Bennett, Labor Secretary-designate Ann Dore McLaughlin, Rep. Bob Michel, and Sens. John Warner and Bill Bradley, brought their families and gifts for the patients at Children's Hospital. The children and grandchildren of the establishment were dressed in their holiday finery, with all the Esprit and Guess regalia exchanged for miles of velvet, lace and taffeta.

Besides Mandrell, the performers for the show, which will be shown Dec. 21 on Channel 4, were Jack Jones, Marilyn McCoo, the Vienna Choir Boys, the Brunson Brothers, violinist Stefan Milenkovic, the Shiloh Baptist Church Choir, the Naval Academy Glee Club and the Army Herald Trumpets.

The Reagans appeared to be relaxed and exuberant. The president was wearing a navy suit and a tie festooned with Christmas trees, and the first lady wore a black velvet suit with a white lace blouse and pearl necklace and brooch. Before taking his front-row seat, the president could be seen peering from behind a red curtain. While he was waiting for the signal to come out, the president poured eggnog for his wife and George Stevens Jr., the show's executive producer, Stevens said. "He poured and we drank a toast to his treaty."

Later, while looking at NBC correspondent Chris Wallace, Reagan jested, "It is a pleasure to see you when you don't have to shout a question."

During the show, Wallace introduced a taped segment of Nancy Reagan in the White House library discussing the history of White House Christmas cards. "I think I have saved every card sent to me by my children, my mother and father, and old friends I haven't seen for a long time," she says. She shows the cards sent by the Franklin Roosevelts, the Eisenhowers, the Kennedys, the Johnsons, the Nixons, the Fords and the Carters. The Reagans have sent cards created by American artists depicting the White House.

"That little light you see is my little room where I am busily wrapping Christmas presents," Mrs. Reagan says, pointing to a Jamie Wyeth work showing a darkened mansion with one light burning. Another, also by Wyeth, has squirrel tracks in the snow leading up to the White House. "We bring in nuts from Camp David and put them in front of the Oval Office door so {the squirrels} can feed themselves."

Milenkovic, a 10-year-old violinist from Belgrade, was introduced by Mandrell as the "Paganini of our age." He played the "Ave Maria" alone; originally his mother had planned to accompany him on the piano, but her music was misplaced.

Reflecting on the spirit of the evening, Sarah Brady, who accompanied her husband Press Secretary James Brady, said, "There was a real feeling of tranquillity and I was so glad Barbara Mandrell brought it up. This sort of starts the Christmas season. Now I'm going to go home and start wrapping Christmas presents."