When the Defense Department begins imposing new drinking age restrictions on military posts on Saturday, most area bases will be -- or will expect soon to be -- exempt.

One of the bases where an exemption permitting drinking as usual has been applied for is Fort Myer in Arlington, where the Army's plans to open a liquor "package store" have particularly irritated county officials.

It was an auto fatality involving drunk driving by a soldier stationed at Fort Myer that led to a recent federal court judgment of almost $1 million against the U.S. government and an out-of-court settlement of $250,000 with survivors of an accident victim.

The judgment and Defense Department concern about an increasing number of drunk-driving fatalities involving military personnel led the department to declare earlier this year that, as of June 1, all military bases must adhere to state drinking-age laws.

But the edict also said the services could seek exemptions for installations that are within 50 miles or an hour's drive of a jurisdiction where the drinking age is lower.

That exemption has sent local bases, particularly in Northern Virginia, scurrying for permission to adopt the more liberal drinking-age laws of the District of Columbia, where 18-year-olds are allowed to drink beer and wine. Hard liquor is sold only to those 21 years and older.

In Maryland, the minimum drinking age for all alcoholic beverages is 21. In Virginia, effective July 1, patrons must be 19 to drink beer and 21 for wine and distilled spirits. By 1987, the minimum age for consumers of all alcoholic beverages in the state will be raised to 21.

Most states have tightened their drinking-age laws recently in response to federal threats to cut off sorely needed transportation funds. A measure that would raise the beer-drinking age to 21 in the District of Columbia is being considered by a City Council committee.

Spokesmen for the area military installations that are seeking, or have received, exemptions said yesterday officials are worried that their young personnel will flock to District bars to drink and drive back to the bases drunk.

"We want to keep them home, control them and provide an atmosphere where they don't see the need to drive 25 or 50 miles away to drink beer and wine, and drive back drunk," said Lt. Col. John Myers, spokesman for the Military District of Washington (MDW), which oversees various Army installations, including Fort Myer.

He cited 1984 statistics showing that alcohol-related deaths involving drivers under 21 rose 18 percent over 1983 in the Washington area while they declined 13 percent nationally.

Arlington County Board Chairman John G. Milliken sent a letter to Col. John H. Virrane, MDW's deputy chief of staff for personnel, yesterday saying the board will not support MDW's request for Army permission to open a liquor store at Fort Myer.

In a letter to the County Board, Birrane had asked for its "concurrence" in the plan to open the liquor store. He wrote that the post would use revenue from liquor sales at the store to support its child-care center, libraries, and several recreational programs, which are strapped for funds.

Milliken, who got the unanimousagreement of the board Tuesday night, cited the disparities between post and Virginia drinking-age laws, the county's concern that it might be held liable for deaths or injuries resulting from drunk driving if the county condoned the liquor store and the potential for losing revenues that comes from sales at state liquor stores.

The Virginia Alcoholic Beverage Control Board, which operates all liquor stores in the state, has a store on Pershing Drive directly across Rte. 50 from Fort Myer's Henry Gate. That store netted a profit to Virginia of over $1.5 million in fiscal 1984, an ABC spokesman in Richmond said.

Myers said Arlington's opposition would not necessarily kill the plan for the store. But he said it "would obviously have some influence in the overall decision-making process."

The requests for exemptions have angered Lou Herzog, a retired Navy captain who is president of the Northern Virginia chapter of MADD (Mothers Against Drunk Driving). "We're not going to put up with it," he said. "They're going to have underaged kids drinking on base, getting drunk and driving off and killing people and themselves. There's no way the military can guarantee that won't happen."