At one point in the 72-hour negotiations over the departure of Soviet ballerina Ludmilla Vlasova, a Soviet diplomat turned to his American counterpart and said: "I hate to be rude, but . . . "

"If you hate to be rude, don't," Donald F. McHenry interrupted quietly. That was the closest the negotiators came to exchanging harsh words, McHenry said in an interview yesterday.

The restraint is typical of McHenry's style. Few diplomats are as low-key while being forceful or use words with as much precision.

Behind his desk at the U.S. Mission to the United Nations hang the presidential commissions for the two general assemblies at which McHenry, 42, has represented the United States as a deputy ambassador and one of Andrew Young's top assistants.

The first cites McHenry's "integrity, prudence and ability." The second omits the word "prudence." It is an inappropriate, undoubtedly accidental, change. Unlike Young, McHenry personifies prudence -- which may be one reason he is being considered as Young's replacement.

When reporters at Kennedy International Airport asked McHenry if the United States blundered in holding up Vlasova's plane for three days only to have her choose to return to Moscow, McHenry answered that the principle of freedom of choice had been upheld and was worth upholding.

When the same question was shouted again in slightly different words, McHenry simply ignored it.

His weekend at Kennedy brought McHenry to public attention as none of his other assignments has.

When word came that Vlasova -- whose husband Alexander Godunov had defected two days earlier -- was ready to leave the United States, McHenry rushed to the airport from the U.N. Security Council. He had already put in a long week there working behind the scenes to avert a Security Council vote -- and promised U.S. veto -- on a resolution supporting the Palestinians' right to self-determination.

Television cameras followed his every movement as he and others involved in the negotiations moved back and forth at Gate 10, as visible as animals in a zoo cage. At one point, reporters appeared on the verge of following McHenry into the men's room.

The lack of privacy was only one problem. During the airport negotiations, McHenry was frustrated and at one point turned to his superior.

"I was out of ideas and I tried desperately to contact Young," McHenry remembered, but the telephone system that tracks down roving U.S. officials all over the globe wasn't functioning. Young was at home in New York, but his phone was broken.

During the airport standoff, McHenry made time for the only part of his life that competes with his job -- his children. One of his two teen-age daughters visited him at Gate 10, and they held a long conversation during a lull in McHenry's talks with the Soviets. They were watched by the crowd of reporters who wondered at first what part the young woman was playing in the drama.

McHenry also talked with his daughters from time to time on the telephone, between phone conversations with Washington. He says that he always takes the children's calls no matter what he is doing or where he is.

McHenry's high visibility at Kennedy airport has helped keep him on every short list of successors to Andrew Young. The blast of publicity has left him unruffled, but it was an unusual experience for a man who prefers quiet diplomacy. Even over drinks and dinner with friends, McHenry expresses himself instinctively with the care and precision of a man giving a well-prepared briefing.

Born in St. Louis, he took his undergraduate degree at Illinois State University and master's degrees in both international relations and speech at Southern Illinois University. He also taught rhetoric and coached the debating team.

After moving to Washington in the early '60s, McHenry studied international relations at Georgetown while teaching English at Howard.

He joined the State Department in 1964, and after almost 10 years resigned in 1973 when Henry A. Kissinger became secretary of state. McHenry was at the Carnegie Endowment for International Peace and the Brookings Institution until returning to government in the Carter administration.

McHenry paused when asked what he does when he's not working. "For the last two years there hasn't been much time when I'm now working," he replied softly.

He is divorced and his daughters live with him in New York while his son is working as an apprentice chef in a Boston restaurant. "Four years of Amherst and graduate study at Oxford are the most expensive preparation for a chef I can imagine," he remarked with a smile.

He is supervising at long distance the restoration of a house near the Calvert Street Bridge, and when he has time, he shops for furniture or fittings for the house. Three weeks ago, he found some old spindles for the staircase railing and he recently turned up an oldsliding door to divide the living and dining rooms.

"I'm going to have to find something that's a better diversion," he said drily.

McHenry is planning to take a couple of days off before the Sept. 18 opening of the General Assembly that has brought him a third presidential commission to hang on his wall. He hasn't yet had time to unroll it from its tube to see whether he is praised for his prudence this year -- an understandable oversight during the past two weeks.

Throughout the first of those weeks, Young had been in the spotlight, chairing the Security Council in his last major activity at the U.N. before his resignation takes effect. McHenry sat in the row behind Young, occasionally leaning forward to exchange a few words and huddling with the outgoing ambassador during breaks.

It was a more typical role for McHenry. Young's ambassadorship has been closely watched by the media, while McHenry has drawn attention only occasionally -- until his name surfaced as a possible successor to Young.

It's just speculation," McHenry said. "I don't expect to be offered it." McHenry has no political constituency and would be a surprising choice, but the fact that he is mentioned as a candidate is a tribute to the quality of his work under Young.

Diplomats at the U.N. and in Washington who know McHenry agree without hesitation that he would be successful as the top U.S. official here.

McHenry made it clear that he would like to stay in his present post if he doesn't get the top job. Or at least he seemed to. But then his careful nature asserted itself. It would depend, he said, on the conditions being right; and Young's successor should have a say in staffing even though McHenry is a presidential appointment.

"I find Andy fascinating to work with," McHenry said. "The private Andy is not the public Andy. He's a very cautious, conservative man, which is hard for the public to believe, but anyone who has sat down with him and talked would say that."

Young didn't tell McHenry about his secret meeting with the PLO representative that led to his resignation. McHenry isn't surprised by that. "If Andy decided to do something like that, he would want to make sure that if there was heat he would take it -- alone."

McHenry didn't know Young before they came to work together. Because both are black, many assume that they came up through the civil rights movement together and have similar backgrounds. It is an assumption that sometimes annoys McHenry.

Throughout his career, he has been at pains to demonstrate the breadth of his abilities and interests. He has no background in the civil rights movement and never taught in the South, although several profiles of him have invented such experiences.

McHenry spends a lot of his professional time on the Mideast and Southeast Asia as well as Africa. This year, he pointed out, he handled disarmament and outer space as well.

"It still tends to come out as 'McHenry the Africa expert,'" he said with a smile. "Life will put you in a narrow channel if you let it."

When he was at the Carnegie Endowment in Washington before joining the Carter administration, McHenry directed its humanitarian policy studies. He deliberately chose subjects with no relation to Africa, McHenry said.

Still, his most intricate negotiations and, in his word, "creative" achievement, was to set in motion the process for Namibia (Southwest Africa) to be independent after 59 years of South African control.

McHenry has consistently deflected credit for the Namibia arrangement to Young. "Andy got us the audience. Andy got us the cooperation. Andy got us the dialogue. Without that, I could have sat at Carnegie or Brookings and written all kinds of proposals. If you can't sell them, it's all academic," McHenry said yesterday.

It was McHenry, however, who led the European negotiators and chased over Africa lining up support from the diverse African parties.

Young and McHenry quickly began to work well together after arriving at the United Nations. Young appreciates McHenry's skill at presenting a briefing or persuading a reluctant diplomat to change his mind.

When Young was invited to appear on a television or radio program while some negotiations were at a particularly touchy stage, McHenry said, Young would often get McHenry to take his place. "I guess he figured that I'd put on my academic cap and analyze it to death," McHenry said.

On a few occasions, Young has called McHenry "the professor," but McHenry said it is not an often-used nickname. "Of course, I don't know what he calls me when I'm not around," McHenry added quickly, grinning.

It's an awkward time for the U.S. Mission to the United Nations to be changing its leader, McHenry said, Articularly because the next General Assembly session may be the most difficult in years for the United States.

Difficult negotiations, however, are becoming something of a McHenry specialty.