At a time when almost no federal penny is too sacred to pinch, Interior Secretary James G. Watt wants to double the tiny budget of a National Park Service program. It would be a bargain at twice the price, because it uses volunteers.

At the Agriculture Department, where budget-conscious officials have sternly pruned away scores of farm publications, Secretary John R. Block found $30,000 and a spare U.S. Forest Service employe to help turn out a new one: a training manual for volunteers.

At the Small Business Administration, where budget cuts have sliced deeply into loan programs, management officials are counting on a volunteer corps of business counselors to put their mouths where the government's shrinking supply of money used to be.

Inspired partly by President Reagan's warm support and partly by the Office of Management and Budget's cold heart, agencies across the federal spectrum have started regarding volunteers with born-again enthusiasm.

"We've beefed it up in the last two years," park service spokesman George Berklacy said of Interior's 12-year-old Volunteers in the Parks program. "It's a reflection of the emphasis on volunteerism."

It's also what the regulators call cost-effective.

Consider: The park service's 12,000 volunteers worked 600,000 hours last year, most of them as visitor guides--a service that otherwise couldn't have been provided. Their cost to the government was a mere $250,000, which covered meals, transportation and the period costumes worn by guides at some historical sites. Watt, apparently counting on even more volunteers, wants to double that to $500,000.

Elsewhere in Interior, the Bureau of Land Management, which never used to keep books on such matters, totted up the figures on the unpaid labor it used last year to help improve range land and recreation areas. It calculated that 1,400 volunteers put in a total of 140,000 hours, or $960,000 worth of work at a cost to the government of only $230,000 in materials and transportation, according to BLM's calculations, and a bargain in anybody's book.

USDA officials hope to get similar benefits out of their pilot program, undertaken in cooperation with the Appalachian Mountain Club, a nonprofit conservation group based in Boston that has helped maintain trails in New Hampshire's White Mountain National Forest for nearly 50 years.

"They're very up front about what they're trying to get out of it," said Chris DuBois of the conservation group. "They want a better working relationship with volunteer groups. Especially with the budget cuts, they have to look at volunteers to do their work."

Without question, volunteers provide the ground-level elbow grease that keeps many a government program in operation. The Health and Human Services Department and Action have identified 760,000 volunteers involved in their programs, according to the President's Task Force on Private Sector Initiatives, which blandly called it "one of the most successful techniques for increasing service delivery while limiting expenses."

But even in an administration that has made a cult of limiting expenses, the immediate prospects of substantially expanding the use of volunteers are not good. The law forbids the use of volunteer services at all but a few agencies that have persuaded Congress to grant exemptions.

Legislation that would have permitted more use of volunteers was in the hopper this year, but didn't rise to a hearing. "It's not very sexy legislation," a congressional aide explained.

Another Hill aide added, "It was so broadly drafted that Exxon could volunteer to run the Federal Energy Regulatory Commission." It was thoughts like that that prompted the prohibition in the first place.

The idea of more volunteers in government is met with skepticism in other quarters as well, notably among federal employe unions, who are defensive about any move that might drive more government employes out of their jobs.

"It raises the same questions that contracting-out does," said a staff member of a House Post Office and Civil Service subcommittee, referring to OMB's policy of turning over to contractors a variety of government tasks that it thinks private industry can do more cheaply.

But sponsors, counting on the keen interest of the White House, hope to advance a bill next year that would let every executive agency, except the military and the Postal Service, use up to 1 percent of its administrative budget to support the work of volunteers.

"Let's try to stretch the dollars we've got," said an aide to one congressional supporter. "In every case where these programs have been tried, they've been successful."

At SBA, which has used volunteer businessmen in a counseling program since 1964, management assistance director James Thomson calls the 12,000 volunteers a "tremendous resource" and says the agency is using them more and more to help fledgling and faltering businesses before they get to the point of needing a loan.

"I have to think it's extremely beneficial," said Thomson. "Dollar for dollar, you couldn't find anything more reasonable."