Arkady Shevchenko, the highest ranking Soviet official ever to defect to the United States, will be sworn in as an American citizen today in the Senate Room of the Mayflower Hotel. The former undersecretary general of the United Nations, Shevchenko will be sworn in by Judge Alex Kozinski, himself a Romanian e'migre'.

Shevchenko, who was a spy for the United States for three years before he defected, said he will be joyful at his new citizenship, but that he misses the Soviet Union. "I miss first of all my own environment. I cannot go to the theater . . . I cannot go to the Bolshoi [ballet]. I miss the communication with my old friends. I just miss the country."

Among the guests expected for the swearing-in -- in addition to Shevchenko's American wife of seven years, the former Elaine Jackson -- are Sens. Daniel P. Moynihan and Malcolm Wallop; Reps. Les Aspin, Dick Cheney and Tom Lantos; former assistant to the president for national security affairs Richard Allen; Robert Georgine, president of the building and construction trades department of the AFL-CIO; and Washington Times Editor Arnaud de Borchgrave. End Notes

TWA thinks it may have run the longest contest in history and is entering it with the Guinness Book of Records. Airline officials have presented Helen Thomas, 80, of Cambridge, Mass., with $50,000 as the winner of the "Cosmic Contest," which dates back to 1955. People boarding TWA flights that year were asked to predict what air travel would be like in 1985. The approximately 13,000 entries were put on microfilm, filed and forgotten until contestants began writing a few years ago to see if they had won. Thomas, who had also forgotten about the contest until she was informed she had won, accurately predicted 5,000-mile flights at 700 miles per hour and emergency helicopter landings on hospital roofs. Three runners-up have yet to be found. It's almost as good as having an unknown relative pass on, leaving a small fortune . . .

Dossier Magazine is expanding its empire. In March a new trade publication titled "Food & Party" will be published, focusing on expensive, flashy entertaining and catering. The first issue of the national publication, initially scheduled to come out quarterly, will highlight Washington, with the feature article naming some of the top social and charity party planners here. The second issue will focus on the New York area, followed by Florida and the Southeast and then the West Coast. With a controlled circulation of 50,000, the new publication is aimed at corporations and association meeting planners. Bill Thomas is editor and David Adler the publisher, just as in Dossier . . .

Washington is apparently a good place for filmmakers. Florentine Films is opening a Washington office. It is the production firm of filmmaker Ken Burns, who has produced several highly praised documentaries, including "Brooklyn Bridge," which was nominated for an Academy Award, and "Statue of Liberty," which has been nominated this year for an Academy Award in the Best Feature Documentary category. His newest film, "Huey Long," is in general release and is presently playing the Key Theatre in Georgetown. Burns said he is opening his Washington office for the proximity to the material for his next major work, a five-part series for PBS on the Civil War. Much of the filming will take place in and around Washington . . .

Rep. Fred J. Eckert is not one to hide his feelings about Libyan leader Muammar Qaddafi. While listening to his car radio on his way to work, Eckert learned that WAVA was giving away floor mats with the colonel's picture on them. He had to have one, and now the floor mat is at the door to Eckert's private office. "It sums up my feelings nicely," he said yesterday . . .

Maybe all powerful women look alike. A young woman television reporter rushed up to Supreme Court Justice Sandra Day O'Connor yesterday at a House Appropriations subcommittee hearing. "We're here to do the interview," the woman announced to the puzzled O'Connor, and added, "Didn't they tell you about the interview?" "Perhaps you better talk to Toni House," the court's public information officer, O'Connor suggested. After some conversation, House asked just who it was the unidentifed reporter was supposed to interview. She answered: "Governor Collins." House smiled and said, "You have the wrong famous woman. That was Justice Sandra Day O'Connor, not [Kentucky] Gov. Martha Layne Collins." . . .