The Fairfax County Board of Supervisors voted yesterday to cut off county funding for the Health System Agency of Northern Virginia, accusing the agency of "propagandazing" on such issues as abortion and smoking.

After a bitter debate, during which board chairman John F. Herrity, who initiated the funding out, was accused of "demagoguery" and compared to the French revoluntionary leader Robespierre, the board voted 5-to-4 to withhold about $27,000 from the health agency.The agency receives about 94 percent of its $635,000 annual budget from the federal government.

Just last week, in approving the county operating budget for fiscal 1979, the board approved funds for the agency, which reviews proposed new facilities and provides long-range health planning.

Earlier this month, the board had asked the health agency to justify what the board called "lobbying" for abortion and antismoking regulations. The agency denied in a letter that it had undertaken any "lobbying effort" and noted that it is mandated by federal law to "educate and inform the general public and public officials about community health questions."

Dean Montgomery, executive director of the health agency, attributed the funding cut yesterday to the antiabortion lobby in Northern Virginia. The pressure is applied by "a relatively small group of moral absolutists that through unrelenting political pressure attempt to force their views on others," he said.

Herrity, who is seeking the Republican nomination for Northern Virginia's 8th Congressional District seat and who is a longtime opponent of abortion, said the health agency "is telling us it is pretty much none of our business how they conduct their business."

Herrity said a government agency "should not be involved in lobbying or educating the public to do anything," and he accused the board of directors of the agency, 10 of whom are appointed by the Fairfax County supervisors, of "believing evangelically in what they are doing."

The health agency, which has been confronted on the abortion issue by the Alexandria City Council and the Prince William County Board of Supervisors, recommended in its annual report "that the governor and the Virginia General Assembly approve the use of public monies for abortions for Medicaid-eligible women."

Laura McDowell, board chairman of the Health Systems Agency, said the agency has taken no stand on abortion and that its recommendation is based on the argument "that since abortion is medically and legally sanctioned, it shouldn't be denied because of lack of money."

The state of Virginia is now under a federal judge's order to provide "necessary medical therapeutic abortions" for indigent women under the state's Medicaid program. That order came out of a pending class action in behalf of all Virginia women brought before U.S. District Court in Alexandria. The Virginia legislature voted in March to cut off payments for abortions unless the mother's life is in danger.

In the board of supervisors' long and emotional debate over the health agency, Supervisor Warren I. Cikins (D-Mount Vernon) said Herrity was "politically motivated"in seeking the funding out and that the board chairman was "fanning the flames of emotion" over the abortion issue.

Calling it "an issue of freedom of speech," Cikins said the board is electing its appointees throughout the country that "they had better watch out because if this board doesn't like what you are doing we are going to punish you."

Montgomery, executive director of the agency, said that if the supervisors are not limiting freedom of speech, they are "certainly making it more expensive."

The funding cut of $27,000 represents about 4.5 percent of the total budget of the health agency, and Montgomery says the out "is obviously not a major element in the budget." But he said the county funding cut will cancel federal snatching funds of about $20,000.

In Alexandria, the City Council voted recently to cut back funding for the health agency from $5,700 to $1,000. In Prince William County, the board of supervisors voted to withhold its contribution if the health agency continued to support abortion funding, according to Montgomery.