Once a week Marie Marschall, the wife of a retired admiral, leaves her home in Alexandria's Cloverway section, a neighborhood of $120,000 homes, and drives 10 minutes to her favorite store to buy food for her family.

Unlike the grocery bills of most Washington area residents who struggle to stretch paychecks to cope with food price increases of almost 15 percent in the last year, almost one-third of the Marschalls' food bill is subsidized by federal tax dollars.

Marschall's favorite store is the giant Cameron Station Commissary in Alexandria. The Marschalls are among the estimated 500,000 area residents eligible to shop in commissaries, the military-run equivalent of grocery stores.

Also eligible for commissary privileges, according to Defense Department officials, are 3,146 diplomats "of foreign countries recognized as friendly to the United States." Diplomats from Soviet bloc countries are ineligible to shop at commissaries.

One of the most controversial and cherished perquisites of military life, commissaries sell cut-rate groceries to the nation's estimated 8 to 12-million active duty and retired military personnel and their families.

Criticism of the century-old commissary system evokes impassioned responses from military families such as the Marschalls, who save an estimated 25 to 30 percent on food costs.

Military personnel defend the system as a well-deserved fringe benefit promised them when they joined the service.

"In 1946 when I joined the service I was told that certain things would happen if I lived through 20 years and didn't get killed in combat," said a retired Army colonel who is now a lobbyist and prominent local official. "I'm entitled to commissaries. It's part of the deal."

Critics charge that the commissary system is, in the words of one Senate aide, "little more than a blatant ripoff which has absolutely no defense value." The criti contend that this has been particularly true in the dozen years since Congress instituted pay comparability, raising low military salaries to the level of similar civil service jobs.

Although the General Accounting Office and Presidents Ford and Carter have favored eliminating the taxpayer subsidy -- $330 million this year alone -- Congress has repeatedly killed such proposals. Sponsors of the bills, introduced in Congress frequently since 1975, have included Sens. Thomas Eagleton (D-Mo.) and John Culver (D-Iowa) and Rep. Les Aspin (D-Wis.).

"You'd think we were paying these guys $100 a month and a cup of coffee," said Aspin aide Warren Nelson. "People just bitch and scream whenever there's a proposal to eliminate the subsidy. It used to be that commissaries were compensation for low pay, but we changed the whole basis for that in 1967 when pay comparability was instituted."

Consider the following:

Despite declining enlistment, the Army Times Publishing Co. reports that 55 new commissaries will be built during the next four years in what is described as a "building boom."

Washington has eight commissaries, more than any other metropolitan area, including the world's largest commissary at Cameron Station.

In addition to the annual multimillion-dollar taxpayer subsidy, the nation's 279 commissaries operate rentfree on federal land and are exempt from state and local taxes. Virginia officials say that in 1977, that state alone lost about $25 million in revenue to commissaries.

"Commissaries were started back (in the 19th century) when there was no corner grocery store in the middle of Wyoming territory." said Nelson. Currently, Nelson said, nearly half the nation's commissaries are within a 10 minute drive of a supermarket.

"There's no economic justification for commissaires in any metropolitan area, especially this one," said Richard Lieberman, a veteran staff member of the Senate Appropriations Committee. "But every three years the Defense Department certifies a need for commissaries based on distance and the fact that food prices are unreasonable. Well, food prices are unreasonable for everybody."

According to Lieberman, "the members of the Senate Appropriations Committee will fight harder over this issue than anything related to military readiness. Why? Well, these retired military have very strong political connections."

Officials of the nation's military retiree organizations and wives' auxiliaries concede that they are listened to on Capitol Hill.

"The military is a target now because it's peacetime," said Marschall, a member of the Washington-based National Military Wives Association Inc. "But commissary shopping is a way of life and Congress knows it can't take that away from us."

c. a. m/cKinney, government affairs director of the 189,000 member NCO (Non-Commissioned Officer) Association, agreed. "In most offices on the Rill they're not attuned to what is going on in the military so we inform them. When commissaries get attacked we get boxes and boxes of petitions from all over the country and we let Congress know."

Like McKinney, defenders of the system say commissaries are an Important inducement to recruitment and the retention of career military families who save hundreds of dollars each year on food.

"I've worked 21 years for this and that includes three tours of duty in Vietnam," said Army Capt. Bill Altman, 40, who will retire from the military this year. "The guarantee when I went in was that I'd get certain benefits and that includes commissaries. It's part of the deal and I don't see any justification for taking that away from us."

Critics point out that abolishing the subsidy would not phase out the commissaries themselves. Even without the subsidy, they say, commissary shoppers would save 10-to-15 percent on food bills.

"That's just not enough to make commissary shopping worthwhile," said Nancy Tucker, editor of Military Market Magazine. "Sure there's savings but you get migraine headaches from standing in those lines waiting for an hour to check out."

Despite 90-minute waits to check out of Cameron Station on a recent Saturday, few customers were complaining.

"You can see why they're patient," said commissary officer Sidney Powers, referring to cut-rate prices of the nearly 7,000 items, including gourmet merchandise, that Cameron Station stocks.

At any one time, Powers said, commissaries feature "wise buys": 50 or 60 items priced 23 percent below the already cut-rate commissary price.

Amount the estimated 2,000 military personnel each day who endure Cameron Station's long checkout lines is Judy Kowal of Alexandria. Kowal is married to an Army staff sergeant who carns $1,110 a month.

"We're on a very tight budget," she said wheeling her three young children and two packed shopping carts through the crowded commissary. "We probably save a third on food by shopping here. I don't know what we'd do without it. We both have partime jobs because military pay doesn't go very far in an expensive area like Washington."

Critics point out that one quarter of commissary shoppers -- and the system's most vocal supporters -- are retirees, many of who could afford to do without the privilege. Congressional studies show that more than 140,000 retirees are "double dippers" who earn military pensions as well as holding second-career federal jobs.

"These guys are making money hand over fist," said Lieberman, noting that their average retirement age is 42 and their average rank is lieutenant colonel.

According to Leo Rothenberg, director of the Federation of State Tax Administrators, state and local governments lose millions of dollars each year in potential tax revenue because commissaries are exempt from state and local sales and excise taxes. Last year commissaries recorded domestic sales of $2.6 billion.

Statistics compiled by the Army Times show that domestic military consumption of tobacco and alcohol, which are tax exempt in commissaries, is 93 percent higher than the national average. For example, Cameron Station shoppers can save about 50 percent on a carton of cigarettes.

In addition to the large commissary at Cameron Station, others in the Washington area are at Andrews Air Force Base in Prince George's County, Bolling Air Force Base and Fort McNair in Washington, Walter Reed Army Medical Center, Fort Myer in Arlington. Fort Belvoir in Fairfax County and Quantico Marine Base in Prince William County.

"There's no question that cigarettes purchased at commissaries are being sold to civilians on the outside," said Rothenberg. "At some military bases per capita consumption of cigarettes is just astronomical."

Defense Department spokesmen say that anyone caught selling merchandise is subject to criminal prosecution, military court martial or loss of commissary privileges or both.

Although commissaries are fourth in Washington area supermarket sales, representatives of Safeway and Giant here and food chains in the Tidewater area of Virginia, which has a large military population, say they have not supported the periodic congressional efforts to abolish the subsidy.

"We don't have any strong objections to commissaries," said Gene Walters, president of the Norfolk-based Farm Fresh Supermarkets. "After all, you can't take something away from somebody that they've had all along. I don't think it would be suicidal for business to oppose commissaries in Tidewater, but it probably wouldn't be the best thing for our public relations." CAPTION: Picture 1, Cameron Station Commissary customers unloading market baskets, at checkout, By Margaret Thomas -- The Washington Post; Picture 2, Commissary price appears at top of carton, above suggested retail price; Picture 3, Commissary customer John Henderson, near the end of a two-hour wait in line. By Margaret Thomas -- The Washington Post; Graph, Saving at the Commissary, By Dave Cook -- The Washington Post