Members of the Fairfax County Board of Supervisors got some discouraging advice today when they came to the General Assembly to argue against ending a moratorium on annexations that the Board contends could lead to the "balkanization" of their county.

Del. Thomas J. Michie (D-Charlottesville) warned the Board members that they had better support the measure that he sponsors to end the six-year-old moratorium because he already has enough votes to win its passage in the State Senate. The measure won approval in the House of Delegates by an overwhelming margin two weeks ago.

County Board Chairman John F. Herrity said after a meeting with Michie he will press the county's five-member Senate delegation to amend Michie's plan in order to exempt Fairfax County. Michie did not indicate he would approve such a move.

A chief argument by Herrity is that if the ban on annexations were removed a number of towns or other communities would seek to enlarge their boundaries, making much of the county's area a jumble of competing jurisdictions.

Today, the county officials zeroed in on a provision of Michie's bill that would create a revenue-sharing fund under which Fairfax County might have to contribute monies to neighboring cities in order to become immune from losing territory through annexation.

Such a fund would cost the county about $333,000 the first year and then probably jump to about $500,000 in future years, according to Robert Fitzgerald, a former state senator who is a lawyer and has represented the county in fighting annexation cases in the past.

Throughout today's meeting, Michie expressed bafflement at the county's opposition to the bill. Rejecting the county's arguments, the delegate insisted that Fairfax will be "no worse off" than it presently is under his plan.

"You're ending the moratorium," complained Supervisor Joseph Alexander at one point. Michie replied that the moratorium "can't go on forever."

Michie conceded that his will may not be the best solution for Fairfax County's problems, but he said it was the best plan that the state could draft this year and said he would not agree to "any significant change" in the proposal. "We can't solve Fairfax County's problems. We know you have got rapid growth . . .," he said

Although Michie did not flatly rule out exempting Fairfax County from the bill, he said he feared such an exemption would create "a loophole through which others can pass" and that he would have to have to further study the measure.

Even though he offered to attempt to work out some of the county's objections to the bill, Michie made clear that his offer was conditioned on Fairfax County support for the measure in the Senate. And he cautioned them that prospects for the bill's enactment "look good" and that the measure has "a lot of appeal" within the legislature.

Fairfax County Executive Leonard Whorton told the county's senators today that the Michie bill "does not recognize Fairfax County for what it is: a government able to deliver" city-like services. For that reason and itssuccess in fighting previous annexation cases, Worton argued the county should be granted immunity from future annexations.

He claimed that if the bill is passed Fairfax County will be forced to seek city status, seek "immunity" from annexations under the law, or take "no action" and wait efforts by others to nibble away at the county's borders. Supervisor Alexander said that the county had no objection to a provision of the bill that would allow the towns of Herndon and Vienna to retain their current status, but other county officials have voiced fears that the towns might later seek city status and annex portions of the county.

Michie said he rejected the county's analysis of his bill because the county was "looking at things from the ideal world Fairfax County would like to have, not the real world." Without the new law, the county "has only one option - to bump along as you are now," Michie said.

The county's opposition to the bill has surprised some legislators here. Although Fairfax County has a full-time lobbyist at the Assembly, some of the supervisors hee today claimed they had no konwledge of an earlier hearing on the measure during the current legislative session. Mayors of both Herndon and Vienna appeared at that session and apparently won support for their towns' status during the meeting.