The old fireplace in the White House had lain dark for years, but yesterday it was suddenly ablaze on the world's television screens as President Reagan and Soviet leader Mikhail Gorbachev hailed their new missile treaty. They crackled with their own enthusiasm, charm and humor on a day devoted to fresh promises to end the nuclear arms race.

They laughed, they poked fun and they celebrated. They paid tribute to their nations' vast differences, but they reserved moments of personal emotion for their joint triumph. Their rhetoric was studded with quotations from Soviet and American literature; Gorbachev borrowed Emerson, Reagan took from Tolstoy. For leaders of two superpowers who had so long denounced the other and who had last met in bitter failure at Reykjavik, they were electric.

As Gorbachev, in front of the blazing fireplace, finished his address explaining the Intermediate-Range Nuclear Forces Treaty in a broadcast to Soviet and American audiences, Reagan clasped his hands in a gesture of delight. As they posed for photographers, Reagan suggested that he and Gorbachev call each other by their first names in their private working sessions. "Mine is Ron," he said, according to an aide. "Mine is Mikhail," Gorbachev replied.

When Reagan repeated his favorite Russian proverb, "trust but verify," in remarks just before the treaty was signed, Gorbachev interrupted, "You repeat that at every meeting!" The Soviets and Americans in the East Room audience broke into laughter and Reagan responded, "I like it."

If the two leaders found reason to wrangle on the first day of the first Washington summit in this decade, they did not show it. In the halls of the White House West Wing, senior Soviet and American officials reveled in a reunion-like atmosphere, U.S. participants said. They had even planned a group photograph, but the meeting broke up before it could be taken. A high-ranking American official observed, "Today was the day of old business -- you move to new business tomorrow."

From the moment that Gorbachev's limousine arrived at the White House, through Gorbachev's remarkable late-afternoon appearance at the Soviet Embassy, the day was filled with signs of optimism and tinged with irony.

Just the tableau on the South Lawn of the White House was extraordinary. Never before in the Reagan presidency had U.S. soldiers been seen carrying Soviet flags (as well as American ones) along the driveway to welcome a foreign visitor. And the tiny, bright red Soviet flags were hoisted along with the Stars and Stripes by guests cheering the arrival of the Soviet leader.

In their first two summit meetings, Reagan and Gorbachev confronted each other gingerly on neutral territory. They briefly shook hands in the cold outside a chateau on Lake Geneva in 1985 and outside Hofdi House in Reykjavik a year ago.

But yesterday they opened the summit drama with high symbolism and full pageantry, both of them vowing in ample terms to move ahead in reducing the superpower nuclear arsenals. The day was a remarkable contrast to the closing of the Reykjavik summit, where the two leaders parted in disappointment over their failure to achieve a breakthrough.

Yesterday they delivered strikingly parallel speeches in a cool, gentle breeze outside the White House. Reagan said, "Our peoples for too long have been both the masters and the captives of a deadly arms race. This situation is not preordained and not part of some inevitable course of history. We make history." Gorbachev responded that "history has charged the governments of our countries and the two of us . . . with a solemn duty to justify the hopes of Americans and Soviet people and of people the world over to undo the logic of the arms race by working together in good faith."

He added, "This will, of course, be the first step down the road leading to a nuclear-free world."

The drama of the day was sharpened at the outset by the chemistry displayed between Reagan and Gorbachev. As Reagan spoke, Gorbachev looked on, nodding slightly in acknowledgment. Reagan smiled through most of Gorbachev's remarks. They stood shoulder to shoulder, too, as the U.S. Army Band, the Third U.S. Infantry Fife and Drum Corps and the U.S. Army Herald Trumpets -- arrayed on the White House steps -- provided a spirited full-honors arrival ceremony. Afterward, in the warmth of the Oval Office, the leaders seemed pleased by the remarkably similar statements they had heard from each other.

"Well, obviously, we want to make progress," Reagan said as they posed for five waves of photographers. "I think both of us made that clear out there in our remarks." Gorbachev said, "You might have noted that there is a great similarity in the outlook of things on the world in our remarks today."

Gorbachev signed a guest book on the president's desk. Reagan presented the Soviet leader with two solid gold cuff links emblazoned with the image of the prophet Isaiah beating swords into plowshares, and said the gift was a symbol of his hopes for arms control, according to White House spokesman Marlin Fitzwater.

Throughout the day, there were unusual greetings and reunions. Former secretary of state Henry A. Kissinger bumped into Gorbachev adviser Georgi Arbatov. Marshal Sergei Akhromeyev, chief of the Soviet general staff, shook hands warmly with Paul H. Nitze, the veteran U.S. arms control adviser who sat across the table from him in Reykjavik -- and again later yesterday as the two sides set up working groups on arms control.

While Reagan was meeting with Gorbachev, Vice President Bush chatted amiably with Akhromeyev about their families and their service in World War II, according to a White House official.

For the signing ceremony, Reagan escorted Gorbachev down the red-carpeted foyer to the East Room, and both leaders expressed unrestrained enthusiasm for the treaty. Reagan quoted another Russian proverb -- "the harvest comes more from the sweat than from the dew" -- and suggested that the negotiators of the treaty "get some well-deserved rest."

Gorbachev took the remark to mean the Soviets should get some rest. "We're not going to do that," he quickly responded to more laughter. The leaders signed the document on President Abraham Lincoln's Cabinet table, trading pens at the end of the ceremony.

As first ladies Nancy Reagan and Raisa Gorbachev watched, the leaders then delivered separate televised speeches from the State Dining Room, standing before the blazing fireplace. As Gorbachev finished, Reagan gestured to a portrait of Lincoln, and Gorbachev was heard to remark that he was a great man.

Later, the Soviet leader was still going strong as he met with American intellectuals. "I said to the president today: 'We have begun a very big thing,' " he said, appealing for cooperation and saying he did not understand opposition in the United States to the missile treaty.

Soviet Foreign Ministry spokesman Gennadi Gerasimov called the day "historic," adding: "Maybe it will be remembered as a date of fame in history." He had called his wife in Moscow, he said, and she reported "celebrations" there. "Everyone was very moved by what happened here in Washington."