Old United Nations hands are signing the institution's 40th birthday card with best wishes -- and a mixture of disillusion and hope.

"We were not naive," said Harold Stassen, one of the original signers. "We didn't guarantee world peace. We said we wanted to build a beachhead to find the way to have world peace."

For the United States, the United Nations' growth from 51 members in 1945 to 159 has meant an often difficult coming to terms with global political changes mirrored in its East River headquarters.

The United Nations "changed character right under our eyes," said Ernest Gross, former U.S. delegate to five U.N. General Assembly sessions. "It was the deadlock between the superpowers that precluded any peace-keeping mechanism with any bite, any teeth."

In the beginning, Gross said, "almost everybody was on our side. The Soviet Bloc was all alone. . . . Now the reverse has set in. There are three times as many members now and they're all over the lot. . . . We don't look so good anymore."

Other U.S. former United Nations officials voice more frustration:

Vice President Bush, ambassador to the United Nations from 1971 to 1972, "regrets that the U.N. has not lived up to its expectations.

"I don't think in that period of time there were any amazing breakthroughs," said Bush of his brief U.N. tenure. "I was enormously frustrated at the U.N.'s inability to be a peace catalyst in the India-Pakistan war. But You'd have to ask yourself, 'Without a U.N. peacekeeping force, would things be worse?' I think the answer is clearly 'Yes,' from time to time."

While he speaks with pride of his role in helping arrange the 1973 Arab-Israeli cease-fire, John Scali, Bush's successor, considers the organization "teetering on the edge of irrelevance. As someone who was a once strong advocate, I find myself disillusioned and discouraged. With some exceptions, its network of agencies have become ones of remarkable ignorance -- separate fiefdoms that often disregard sensible advice or realistic budget control."

But some see the changes as offering a challenge the U.S would do well to take up.

"The basic difference," said John McDonald, deputy director general of the International Labor Organization in Geneva in 1974-78, "is the United States dominated the scene in '45 and now in terms of numbers we don't dominate . . . we get outvoted." But McDonald sees the change as "a stimulus to hone our own diplomatic skills. After all, it's based on a democratic principle. We shouldn't object to it now just because we get outvoted."

And, despite the frustrations, many who came here for the anniversary feel the United Nations has a good deal to celebrate.

"It has proved to be an extremely useful institution," said Argentine Ambassador to Washington Lucio Garcia del Solar, a former ambassador to the United Nations. "But too much hope was established on agreeing on political subjects. You cannot agree on political subjects; they are dominated by strong ideological positions. It is those that separate the countries. That gives the U.N. small possibilities of success in the big political discussions."

"One could feel a healthy wave of rededication," said Brazilian Ambassador and former U.N. representative Sergio Correa da Costa. "It made people realize we actually could not do without it. . . . The simple fact there is a forum where we can air our differences and concerns . . . is something we could not do without. Even if it did not exist, the U.N. would have to be reinvented."