Andres Segovia was in the East Room, but Jimmy Carter was in the Middle East. Thought the twain did not meet yesterday, "if we get peace," said Walter Mondale, "it will all be worthwhile."

It was the fifth and final White House Sunday afternoon concert (until next fall). President Carter's standin, Vice President Mondale, was there as host to introduce the 85-year-old Segovia as the "unchallenged master of the Spanish classical guitar."

And it was the second time in six months that the Israeli-Egyptian peace negotiations had kept Carter off the East Room stage with his guest artist. The first was when he conducted the Camp David negotiations last September and cellist Mstislav Rostropovich was the afternoon's star attraction.

"I know the hopes of all humanity are with the president as he searches for peace in the Mideast," Mondale told the 220 guests, including a crosssection of official and unofficial Washington as well as Amy, Chip and Annette Carter and the president's sister, Ruth Carter Stapleton.

Carter telephoned Segovia in New York to tell him he would not be at the White House concert, Segovia's wife Emilia said yesterday. "I think" she added, "its more important what he is doing now. We need peace -- in Israel, in Spain, everywhere."

Segovia said he was disappointed but that he was glad the president had sent the vice president to fill in.

The way that happened, according to what Mondale told the crowd in the East Room, "I asked what my duties were as acting president. I said [to Carter], 'Would you like me to sit in the Oval Office while you're gone?" And he said, 'No'."

When Mondale asked if he should represent the president in Venezuela and Brazil at the inaugurations of their presidents, Carter said he thought Joan Mondale would do "a better job. Besides, Fritz, I want you to be at the center of power at all times." That was why, said the vice president, he was with Andres Segovia yesterday.

After the concert in the Blue Room, as Mondale and Segovia toasted each other with champagne, Mondale's expansiveness disappeared when questioned about reports from Israel that Carter was extending his stay by a day. "No comment," said Mondale.

Chip Carter, who joined Mondale and the Segovias (with the couple's 8-year-old son) in receiving guests, said he too had heard about the extra day but did not know what it meant. He said he thought that if his father's journery is successful, it will help his re-election chances. If it fails, "it won't hurt -- I don't see how it can. He's done as much as anyone can -- that's my biased thinking, of course."

Chip Carter said the president "went over there thinking it was a chance. He had real doubts and everyone around here was real nervous. It's not the last mile, but it's a good start."

Chip Carter said he has Democratic party "commitments" until mid-April, after which he expects to join his father's re-election campaign committee as a paid "surrogate speaker... I will have to be paid. I've got a little boy to raise." He and Caron Carter continue to be separated, though still "good friends," he said.

Unlike the president's son, who lives in the White House, Evan Dobelle -- who is expected to become coordinator of the Carter campaign committee -- was guarded about discussing re-relection efforts inside the Executive Mansion.

"The law says we have 10 days to file the names of the committee with the FEC (Federal Election Commission) and in the meantime one should not discuss the presidential campaign in the White House," said Dobelle.

Of Carter's bid for peace, Dobelle remained aloof -- "I wouldn't like to put a political tone on it. The main thing is peace in the Middle East, that's the president's main motivation."

Amy Carter, 11, was the first person through the receiving line, turning her cheek for a kiss from Emillia Segovia, whom she had med a year ago when the Segovias attended the first Sunday afternoon concert given by Vladimir Horowitz.

"How's the violin coming?" someone asked her, as she walked past. "Yuk," she replied, making a face.

G. William Miller, head of the Federal Reserve Board, said he was struck by "how soft" Segovia's hand was -- not a trace of a callus. The softness of the economy was another tune.

"This afternoon, it doesn't look soft," he said nooding toward the well-dressed crowd munching cold lobster medallions, prosciutto with fresh white asparagus, hot oysters with Bernaise sauce and other dishes passed on silver trays by white-gloved waiters.

"But," Miller continued, "the psychology of inflation is continuing. People are spending a little more than they should. I think it will slow down a little as the year progresses."

Segovia, clad impeccably in cutaway coat with pearl-gray vest and string tie, sauntered through the mansion in an almost proprietary manner. It was his first performance there, though John Kennedy had invited him several times. "Other presidents weren't very fond of music, I think," he told Mondale.

"That's why I quoted General [Ulysses] Grant," Mondale said, referring to a story he had told everybody earlier in the East Room: President Grant used to summarize his feelings about music with "I know only two tunes -- one of them is 'Yankee Doodle,' the other isn't," said Mondale.

"Fortunately," Mondale added, "the spirit conveyed by that comment is long departed."