For the past two decades, the highlight of a trip to Moscow for art lovers was a visit to the spacious Moscow apartment of George Costakis, the Greek-born Canadian Embassy administrative officer, who had amassed one of the world's largest and most important private collections of Russian avant-garde art. It consisted of 2,000 paintings, sculptures and watercolors by Chagall, kandinsky, Malevich, Rodchenko and others who worked in the decade and a half before and after the Russian Revolution.

Last January, Costakis, his Russian wife and three of his four children left Moscow for Italy, after donating 80 percent of his collection to the Russian people. It will be housed in a new museum of post-Revolutionary art nowunder construction.

At the Smithonsian's Woodrow Wilson Center yesterday, Costakis talked of how he had begun buying right after World War II. "I am the richest man in the world in Rodchenkos, but 90 percent were gifts. Thank God." He stopped collecting in 1961 when prices began to rise "because of the emigration. Now, instead of bringing diamonds out, people are bringing avandgarde Russian art, because if you can get Chagall, Kandinsky or Rodchenko.

The portly, bespectacled Costakis, who looks more like Aristotle Onassis than Joe Hirshhorn, whose buying habits resemble his own, had a big bear hug for nearly every one of the hastily assembled group of fellow emigres and smiling American friends, nearly all of whose knew the Moscow apartment well.

Fred Starr, secretary of the Wilson Center's Kennan Institute, explained, in introducing Costakis, that his collection was important because "it kept intact and made available a whole period of art which could not be seen publicly at the time. The visitors came on a scale comparable to that at Lenin's tomb. Starr and Costakis are currently working on a book about the collection.

After his marriage 46 years ago, Costakis began collecting Dutch paintings, carpets and silver. "Moscow was full of them," he said. "But by accident I found two or three avant-garde works 33 years ago, and it was as if I had lived all my life in a dark room with closed windows. With those paintings, it was as if the walls fell down. It was a revolution in art comparable to none. Nobody understood and nobody appreciated them. I began collecting Malevich for a few kopeks apiece."

At a luncheon following his talk, Costakis said he is currently in the United States "to see about the book and to clear up my American visa problems. My visa has been delayed due to problems not created by officialdom." He is being helped by Sen. Edward Kennedy, Rep. John Brademas, J. Carter Brown and others.

The biggest revelation came via a recent phone call from Costakis' youngest daughter, who continues to live in the Moscow apartment, and he shared the news triumphantly. "She said that she had been called to the Ministry of Culture and was told that there will be an officially sponsored exhibition of Russian avant-garde art in Rome this fall, and that it will trave to the Pompidou Center in Paris in the spring. And most important, there will be a big exhibition in Moscow in 1979."

When asked whether he regretted the loss of all but 20 per cent of his collection, Costakis said, "I have always said that I would leavy my collection to Russia, and I am glad, happy and proud that I have done so. Thank God I got 20 per cent. There was a time that i thought I would get nothing."

"I will sell no more than I need to' to live on," said Castakis, whose only other income will be from his Canadian Embassy pension. "I may give some works as gifts to American of Greek museums, but I really want most of all to keep as much of the collection together as possible."

Several examples from Costakis' remaining share were shown last year in Dusseldorf. They are now stored at the Guggenheim Museum in New York.

Costakis also revealed that he himself has taken up painting during the eight months he has spent in Rome since leaving Moscow, and that his style (unlike the wholly abstract artists he collected for so long is realistic. "I think Andrew Wyeth is a great artist. Maybe young collectors will be soon starting their collectors with a Costakis! I must say I like amy paintings, and I have a lot of laywers dying to get a present from me. One day I will show it, and sell it for $1,000!"

After lunch Costakis left for New York to attend the opening of the Mark Rothko exhibition at the Guggenheim Museum. He will remain there until the end of the month when he returns to Rome to await his visa. As soon as it comes, he will do what many aspiring artists before him have done - move to New York.