LEXINGTON PARK, Md. A long-standing practice at Southern Maryland bars and liquor stores -- selling mixed drinks to go from drive-up windows -- is coming under fire in St. Mary's County in the wake of a grand jury recommendation that the sales be banned.

State police, who say minors and others are being sold liquor at dimly lit windows, contend that the practice, allowed by county law, adds to the number of drunken drivers on county roads and increases the risk of alcohol-related accidents.

In September, a county grand jury attacked the sales of so-called "go cups" there, and charged that it was behind the county's high percentage of alcohol-related traffic deaths and increased drinking by juveniles.

A recent school system survey of teen-agers in St. Mary's indicated that 90 percent drink alcohol regularly, a statistic the grand jury said is 20 percent higher than any other county in the state.

Besides urging that the sale of "go cups" be outlawed immediately, the grand jury report noted that the three-member liquor board had revoked no licenses in the past 3 1/2 years, despite a state police undercover investigation this summer that resulted in 30 bartenders being fined for selling to underage police cadets.

Owners were ordered to appear before the liquor board, and a few had their licenses suspended briefly.

"But most were not given a penalty of any kind, although some establishments had been cited twice for selling to minors," said Lt. Richard Stufft of the state police barracks in Leonardtown.

"One of the main problems is the liquor board's lack of accountability," said Herbert Winnik, a history professor at St. Mary's College who was foreman of the grand jury. "We looked at all their records and found no violations or complaints of any kind documented."

"The county has a long tradition of alcohol use, but serving these 'go cups' is playing with fire," he said.

Shortly after the report was issued, two of the three liquor commissioners -- who are appointed by the governor -- submitted their resignations. One member, Frank Wilkenson, cited health reasons and the other, Virginia Sapp, said she wanted to devote more time to her teaching job. The remaining member is Gaylord P. Aschenbeck, a retired merchant.

The grand jury undertook its probe of liquor sales because members were concerned "about alleged sales to minors and about the way the liquor board was enforcing the law," assistant state's attorney Joseph A. Mattingly Jr. said. The percentage of traffic deaths related to alcohol use in St. Mary's is the highest in the state.

In 1983, 18 people were killed in 17 fatal car accidents in the county, state police said. In 12 of those accidents, alcohol was a factor, they said. Thus far this year, eight of 11 fatal auto accidents involved alcohol, police said.

"Alcohol is the reason for about 75 percent of the county's fatal car accidents," or more than any other jurisdiction in Maryland, Stufft said.

Statewide, alcohol was a factor in 51 percent of last year's fatal accidents, state police said.

St. Mary's, with an estimated population of 62,000, also has the highest number of liquor licenses per capita of any county in the state, officials said. Of 642 businesses in the county, 194 sell liquor or about one liquor retailer for every 320 people, Stufft said. In Montgomery County, by contrast, where package liquor stores are operated by the county government, there are 607 liquor licenses, or one for every 1,153 persons.

At the same time, arrests of drunken drivers in St. Mary's -- which stand at 432 through October -- are down 10 percent from the same period in 1983, Stufft said. He said increased public awareness statewide and stricter penalites locally may account for the decline.

Last week at a public meeting held by the county's delegation to the state legislature, some liquor retailers, school representatives, parents and others joined the grand jury in calling for an end to the sale of the carry-out drinks.

"This is no longer a sleepy Southern Maryland county," said Edward Monahan, owner of B&B Liquors in Lexington Park. " . . . We have a four-lane highway and need a liquor board that thinks like 1984 not 1934."

Monahan, whose store does not have a drive-in window, said he nonetheless gives customers plastic cups and ice "because my competitors do. You can't walk out on the street with a cup of booze in D.C. I'm just trying to get some control here."

In the District of Columbia, liquor may not be sold at drive-in windows, and drivers caught with open containers of alcohol in their cars may be charged with drinking in public or driving under the influence.

Montgomery and Prince George's counties do not allow the sale of drinks to go from bars or stores. In Prince George's, bars with the right kind of license may sell customers sealed bottles of alcohol over the counter, a spokesman for the county's liquor board said.

In Charles County, restaurants can sell customers individual "go cups" if they are properly licensed, a liquor board spokesman said.

"The fact that the liquor board has not outlawed the 'go cups' makes it appear that citizens have tacit approval to drink and drive," Stufft said when asked about the grand jury report.

State police Lt. Jim Plunkert said he knows of four bars in St. Mary's with drive-through windows. The liquor stores with drive-in facilities sell beer by the can, but aren't equipped to offer mixed drinks, he said.

"It's incredible to think that drunk driving is illegal in Maryland but imbibing these 'go cups' while driving is not -- at least in St. Mary's," Plunkert said.

"It makes my blood boil to think that these stop-and-go places are legal," said Bob Daniels of the Charles County Mothers Against Drunk Driving (MADD) chapter. Since Daniels' 20-year-old daughter was killed by a drunk driver in August 1982, he has fought in vain to get Maryland to outlaw open containers of alcohol in cars.

Daniels, school counselors and local police say the "go cup" practice is a holdover from the days when streams of visitors came to Southern Maryland to play slot machines.

Gambling on slot machines was outlawed in 1968, but "we still have the reputation as a free-wheeling drinking spot," Daniels said.