Smithsonian Secretary Robert McC. Adams said yesterday that the Office of Management and Budget has instructed the institution to plan for a possible 32 percent cut in its fiscal 1991 appropriation, a scenario that would prove "almost unbelievably devastating."

Adams said the institution has not been told to take any action, and he acknowledged that the order may represent some political brinkmanship as budget talks drag on. But, he said, "the only responsible position we can take is that this is a serious possibility and we do need to plan for it." In recent weeks, even before OMB's warning, the Smithsonian had said it faced severe budget constraints.

In a memo to his staff, Adams said the mere threat of such a cut "will cause substantial and unfortunate effects on the morale of our staff." He ordered a freeze on hiring and a halt to any new commitment of federal funds to projects.

The Smithsonian got its orders late Wednesday in a phone call from OMB, which confirmed through a spokeswoman yesterday that all government departments and agencies have been advised to plan for possible across-the-board cuts in case a budget agreement is not reached by Oct. 15. In that case, the Gramm-Rudman law would force $104.8 billion in automatic spending cuts.

"We're in contact ... with all departments and agencies to discuss the need to prepare for sequestration," the spokeswoman said. Privately, other OMB officials said details of the deep contingent cuts could be used to pressure congressional bargainers to reach an agreement when talks resume early next month.

If the plan were to go into effect, the Smithsonian calculates it would be required to trim about $89 million from a $279.3 million departure point set by OMB. In fiscal 1990 the institution got $267 million from the federal government; so far, a House subcommittee has approved $313 million for 1991.

The mere possibility of such Draconian cuts left Adams shaken. "If this is across the board ... it's really hard to know how we can adapt to it," he said. Such a cut probably would mean layoffs, furloughs and closing programs, he said.

High-ranking Smithsonian officials had mixed responses to the news. Spokeswoman Madeleine Jacobs said senior staff members discussed the matter calmly yesterday at an already scheduled luncheon. "Nobody expressed any undue concern because we felt like this isn't going to be the final figure. ... People were very philosophical. They were not overreacting," she said.

Tom Lovejoy, assistant secretary for external affairs, said the prospect of deep cuts is "terrifying." While the final figures may not been as harsh as feared, he said, "one has to at least take it at face value initially and make appropriate plans. ... Frankly, I haven't even had time to think it through. I think everybody's sort of looking a little cross-eyed today. Nobody's even heard a number like 32 {percent} before."

But James Demetrion, director of the Smithsonian's Hirshhorn Museum and Sculpture Garden, said he was not drawing up a doomsday plan. "It's my understanding that there's no assurance that that's going to be the final figure," he said. "... Until it's absolutely definite, I'm just going to wait. Maybe {Adams} has been asked to do this {plan}. I have not."

Adams refused to go into detail about the effects of a major cutback. "What we've done is to carry on extensive discussions about what our core institutions are and what we need to do to protect them," he said. "It would be unwise and premature to speculate what specific steps we would take."

Adams spoke from his Colorado home where he spends each summer, but he said he will talk to his senior staff about the budget at a previously scheduled meeting in Washington next week. "We'll all be crying on one another's shoulders," he said.

Generally, Adams said, the Smithsonian's first priority in the face of deep cuts would be to protect its collections. The institution also would evaluate planned construction and renovation and postpone whatever possible. Adams said a slashed budget would also mean restricted public access to the Smithsonian's museums.

"My guess is we could not handle a 32 percent reduction without reducing the times of public access and possibly taking into account past attendance figures in order to set priorities," he said.

The idea of charging admission to Smithsonian museums, which traditionally have been free, would be pushed to the forefront, but fees would not be imposed quickly or without discussion, he said. "It's a step we would resist taking and a step that cannot be taken quickly," he said. "There are a number of museums which are free by statute. It's a matter on which Congress is very divided."

The Smithsonian will continue to study an African American "presence" on the Mall, Adams said, and the planned American Indian museum will probably go forward, although "the pace ... is certainly adjustable."

While Adams believes that the various components that constitute the Smithsonian should run autonomously, he said a budget crisis would mean more of a hands-on role for him. "Obviously, we're going to have to impose common standards of priority setting," he said. "... I intend to do everything I can to preserve the core activities of the institution."