At the National Wildlife Federation last month, 56 employees were called in to supervisors' offices one afternoon and terminated without notice. Workers who showed up over the weekend to clean out desks found their electronic key cards had been deactivated.

The abrupt and unexpected layoffs -- 7 percent of the federation's 780 employees -- sent shock waves through the organization, the nation's largest and most prosperous environmentalist group.

The federation's problems are not unique. War and recession are taking their toll on an environmental movement that just months ago had seemed all but invincible. While no others have gone to the lengths of the federation, many major environmental groups are bracing for a painful period of belt-tightening -- cutting travel, postponing raises, curbing expansion plans.

"We don't think there's any falloff in people's commitment to these issues, we just think they have less money to spend or are afraid they're going to," said Ben Beach of the Wilderness Society, which recently laid off seven of 135 employees, including four in Washington.

Charitable giving often declines during difficult economic times, and environmental executives say there is no reason to expect their organizations should be immune. Moreover, several suggested that the Persian Gulf War could stimulate new interest in their cause by focusing attention on energy conservation.

Still, the prospect of cutbacks after a decade of unrivaled growth has shaken environmental activists. Oil spills and fears of global warming had driven membership to record highs, and the huge outpouring of public support generated by last year's Earth Day celebration seemed to herald a new interest in environmental issues by mainstream America.

Now, several major environmental groups are reporting that growth in membership and revenues is flat or nearly so. Others are doing better, but executives remain concerned that the situation could change if war and recession drag on.

"It's going to be hard to predict continued growth," said Kathryn Fuller, president of the World Wildlife Fund, which grew from 60,000 members in 1982 to a million last year. "Certainly, right after the war broke out, we saw a drop in {new} membership, but then it picked up again."

At the Natural Resources Defense Council, membership doubled between 1984 and 1990, to 170,000. Now, Deputy Executive Director Frances Beinecke said, revenues and memberships are essentially "flat" and while 1990 "was a growth year," it was "less than we expected."

It was a similar story at the Sierra Club, where spokesman Peter Larmer said the 630,000-member group had begun making cuts in "non-program" areas -- supplies, expense accounts, office equipment -- as "an insurance policy against potential recessionary impacts."

The Wilderness Society's Beach said the organization was suffering not so much from a lack of members -- the group has more than doubled since 1987 -- as in the amounts they are willing to contribute. "Whereas a member might have given twice in the year, he's only giving once now," Beach said.

Moreover, the average donation is "down 12 percent from a year ago," he said.

Nowhere have the effects been more severe than at the National Wildlife Federation, where the annual budget has tripled, to $90 million, in less than a decade. Although revenue at the end of December was up 9 percent over the same period last year, it was well short of the 17 percent the organization had been anticipating, according to federation President Jay D. Hair.

The result was a projected $4 million shortfall in the federation's 1991 budget, Hair said, and the outbreak of war has not helped matters. "Airports were empty, malls were empty and our mailboxes were empty," Hair said in an interview. "To me, it's a very volatile time . . . and not just for the National Wildlife Federation."

In addition to the layoffs, the federation is leaving 54 new vacancies empty and freezing all salaries. Hair said he chose to forgo an increase in his $220,000 annual salary in January, chopped $35,000 from his office budget and canceled a planned trip to Thailand and Vietnam.

"I cannot envision any sector of society not being affected by this," Hair said. "The whole point is that wars are really bad for the environment, the people and the economy."