The Fairfax County Board of Supervisors yesterday voted 7 to 2 to endorse the Reston area as the site for any new hospital built in the county. The proposal was vigorously pushed by Supervisor Martha V. Pennino, who represents the area.

Meanwhile, Virginia State Health Commissioner James B. Kenley yesterday rejected a third proposal by Hospital Corp. of America to build a $36 million hospital in Reston.

Hospital Corp. spokesman Milton E. Capps said the Nashville based for-profit hospital firm was informed of the decision turning down its 226-bed hospital proposal yesterday but had not received a copy of the decision. Capps said Hospital Corp. plans to file an appeal with Kenley today.

Both Fairfax Hospital Association and Humana Inc., another major for-profit hospital chain, are appealing earlier rejections by Kenley of their applications to build new hospitals in western Fairfax.

Kenley has consistently ruled that there are too many hospitals in Northern Virginia and that no new ones are needed at this time.

The Board of Supervisors dropped a controversial aspect of Pennino's proposal that would have required that a hospital be located within 15 minutes of the homes of 90 percent of Fairfax residents.

Regional health planners estimated that stipulation would have required a minimum of two new hospitals in Fairfax, which critics charged would created an oversupply of hospital beds and cause hospital rates to go up.

Annandale Democratic Supervisor Audrey Moore, one of the two dissenting supervisors, said that the Reston site would still mean that there would have to be two new hospitals, since Reston will not be very convenient for those in southern Fairfax.

"I'm very disappointed, because the board is in effect including in its plan two full-service hospitals, one in Reston and one in Centreville," said Moore. Centreville is located in southern Fairfax. "Figures show that that is going to cost the taxpayers a bundle."

The Health Systems Agency of Northern Virginia, a regional health planning group, has said that if there is to be a new western Fairfax hospital it probably should be centrally located to serve both the northern and southern portions of the county.

Fairfax Hospital Assocation wants to build a hospital in the Fair Oaks area, near the intersection of I-66 and Route 50. Hospital Corp. originally selected that area, but then switched to Reston, where Humana also wants to build.