The Fairfax County Board of Supervisors served notice yesterday it will go to court if necessary to fight a new state regulation that puts pressure on the county to adopt a full-day kindergarten program.

Board members and school officials together attacked the full-day kindergarten requirement as a rural-backed regulation that is educationally unsound and would prove costly for school systems in large counties. Kindergartners in Fairfax are in a half-day program.

Dubbed a "teacher employment act" by Board Vice Chairman Martha V. Pennino (D-Centreville), the new measure requires that state school aid be reduced by 68 percent for any county school system that fails to switch to full-day kindergartens by 1982. This funding condition, if unmet, could cause Fairfax County to lose up to $900,000 in its fiscal 1982 budget, school officials said.

Ann P. Kahn, chairman of the county's school board, warned the supervisors that converting the kindergarten program from half to full days could cost county taxpayers some $15 million. It would mean, she said, doubling the current kindergarten staff and increasing capital expenditures to build more classroom space.

In unanimously opposing the measure, the supervisors yesterday likened the state's insistence on full-day kindergartens to an earlier demand by the federal Department of Education that the county establish bilingual programs for its non-English speaking students. The federal agency recently dropped that order following a strong county protest.

"We'll use the same approach as we did on the ESL [English as a Second Language program], and we have the same message to the legislature and the governor," said Board Chairman John F. Herrity (R). "We're not going to take this lying down." He added teeth to a resolution of support for the school board by directing that the county attorney "look into" the legal ramifications of opposing the state regulation.

In contrast with the big counties that would have to hire more teachers and provide more space, the smaller, rural school systems see the regulation as a means of cutting costs, Kahn said. They "are trying to save money by busing once a day. They do not want to incur additional busing costs."

The concept of full-day kindergartens was approved several years ago by the legislature despite objections from Virginia's larger school systems, including Fairfax County. Kahn said the Fairfax Education Association has endorsed full-day kindergartens while the Virginia Education Association has more recently backed off of its support for the idea.

Board members said they plan to send a special delegation to Richmond this legislative session to explain their objections to Gov. John N. Dalton and members of the General Assembly.

In addition to philosophical and educational reservations about the state regulation, Kahn said the insistence on all-day kindergartens particularly worries her because state authorities are interfering with local control of school systems and tampering with traditional funding formulas that are based on school populations, not policies.

Supervisors said yesterday they would be more than willing to incur the cost of converting to a full-day kindergarten system if they believed such a program would be beneficial for the 5-year-olds enrolled in the country schools. But the argued that studies had shown that children that age have the attention span of three hours and that confining them to a classroom beyond that time amounts to "babysitting."

"This is an extremely expensive way to have day care," argued Supervisor Sandra L. Duckworth (D), who represents the Mount Vernon District.