In a four-week burst of legislative fervor, the Fairfax County Board of Supervisors has restricted throwaway containers, magazines like Playboy and Penthouse, people who smoke in public, outdoor water use and sexually active dogs.

Not even the tiny gnat has escaped the supervisors' attention. Last Monday they considered declaring war against what was described as a "plague" of gnats in the county. Weighing the $20,000 cost, they put off action.

The Board hasn't been so active since the early mouths of its term, in 1976, when it set about undoing some of the actions and policies of its more liberal predecessors. At that time, under the leadership of its new Republican chairman, John F. Herity, the Board lifted a sewer morarium in the Dulles-Herndon-Reston area, endorsed a scaled-down 1-66 extension from the Beltway to the Potomac River and began an agressive campaign of seeking new industry.

Those decisions led to the Board being characterized as conservative, but its recent activism suggests, if anything, ideological eclecticism.

Conservatives, apart from libertarians, could rejoice over the ordinance restricting the display and sale of sex-oriented magazines. But environmentally minded liberals could be heartened by the bottle ordinance, which is aimed at reducing roadside litter.

"I can't figure out what's going to come next," said Chairman Herrity, who opposed the throwaway and smoking ordinances. "Some of these things almost remind me of Lyndon Johnson's Great Society program. Are we going to have a war on smoking, a war on litter, and spend a lot of money and their don't achieve anything?"

Similar questions were raised by Supervisor Marie B. Travesky (R-Springfield) even as she was about to vote for the restriction on public smoking: "We are adopting a lot of ordinances, and while I'm not sure we are really infringing on rights, what I thing we are doing is raising the expections of citzens and won't be able to deliver on them."

But Supervisor Alan H. Magazine (D. Mason), who is generally an activist, defended the legislation without hestitation: "There are many times when this Board has wholeheartedly jumped right in and adopted an ordinance because it felt the ordinance was for the health, safety and welfare of the people of the county."

Public affairs director Edmund L. Castillo points out that none of the recent restrictive ordinances "was invented in Fairfax County." Loudoun County, he said was the first major jurisdiction to pass a container ordinance, and Montgomery County led the way on restricting public smoking.

But Fairfax is perhaps unique in its attempt to encourage - planned parenthood in the dog world. Under a new ordinance, the owners of spayed or neutered dogs will have to pay only $5 for a license, while the owners of dogs that aren't so fixed will have to pay $10.

The supervisors resolutely defeated an amendment by John P. Shacochis (R. Dranesville), who sought to exempt dogs of a certain age (10 or older).

Although all the recent activism took place within the month of September, some of the supervisors said it was more the result of coincidence than design. They also pointed out that the month was preceded by a long summer recess.

Some of the most unexpected activism was originated in the office of County Executive Leonard Whorton, who was expected to be less of a mover and shaker than the man he replaced last year, Robert W. Wilson, now administrator of Prince George's County.

On the morning of Aug. 5, hours after the Fairfax County Water Authority was told by its engineer-director, James J. Corbalis Jr., that there was no immediate need for water conservation, Whorton invoked the first phase of the county's just-passed water emergency ordinance and persuaded Alexandria and Prince William County to go along with voluntary conservation.

All through August September, he and his water advisers have warned, in burnt language that drastic steps may have to be taken to keep the Occoquan Reservoir from going dry, and in early September, on his recommendation, the supervisors approved the first mandatroy restrictions in Northern Virginia. So far, his early warnings have been borne out as the reservoir continues to recede.

Under Fairfax's urban county form of government, the county executive had fewer powers than the same officials in Montgomery or Prince George's counties. In Fairfax, the county executive, who is appointed, not elected as in suburban Maryland, is essentially an administrator carrying out the policies of the elected supervisors.

But as Herrity said in commenting on Whorton's sometime activism, "When there's no direction or policy, he calls them as he sees them."