Starting today, most Marylanders must be 21 years old, instead of 18, to drink any alcoholic beverage legally.

Because the controversial new drinking-age law does not affect those who are already 18, 19 or 20 years old, the law's immediate effect will be on those turning 18 today and in the future.

Waiters and waitresses who are not legally able to consume or purchase liquor as a result of the higher drinking age still will be able to serve alcoholic beverages in most jurisdictions, however.

The repercussions of the higher drinking age are many. A court challenge is under way alleging that it is unconstitutional to allow some, but not all, 18-year-olds to drink legally.

Liquor board officials say they will have to work harder at inspections and enforcements and in some counties they have sent details of the new laws to all licensees. Tavern owners and store clerks say they will have to check identification cards more closely because the new law applies to some, but not all, persons in the 18-year-old group.

And some establishments, particularly those in the College Park area that have catered to the 18-to-21-year-old college crowd, anticipate a drop in business and have planned new strategies to attract older patrons.

The purpose of the new law, according to the legislators who voted for it and the lobbyists who pushed it, is to curb drunk driving and auto fatalities by restricting the availability of liquor to persons under 21 years old.

A spokesman for the Maryland State Police, Dan McCarthy, said this week that he expects drunk driving to decrease as a result of the new law.

State police records show that there have been 116 fewer auto fatalities this year than last and McCarthy said laws passed in 1981 to toughen penalties for drunk drivers are partly responsible.

The Rendez-Vous, a popular night spot in College Park that caters primarily to undergraduates at the University of Maryland and serves only beer, is undergoing a face-lift to draw an older clientele because of the change in the legal drinking age.

"It definitely won't get as crowded as it does now," says Eli Kosanovich, one of The Rendez-Vous' managers. In anticipation of the loss of college-age customers, the establishment has been remodeled and a new awning already hangs in front. The owners may apply for a wine license and there will be a more sophisticated menu in the restaurant, according to Kosanovich.

Critics of the higher drinking age, including some legislators and tavern and liquor store owners, say the law will fail to reduce drunk driving among teen-agers because many 18- to 21-year-olds will still be able to purchase liquor in the District and in neighboring states, such as Delaware and West Virginia.

"In our area they will just go into the District to get it," says Gordon Williams, the owner of Lee's Restaurant in Hyattsville and a past president of the Licensed Beverages Distributors of Maryland. "If we crack down, and we have to, they'll just go to D.C. It's a big hassle."

Liquor store owners across the District line acknowledge that they expect their businesses to improve, but several emphasized that they will not encourage 18-year-olds to take advantage of the proximity.

In addition to raising the drinking age, new laws that will go into effect today include the confiscation of license plates of repeat drunk drivers for 120 days, a requirement that drunk drivers submit to a chemical blood test for alcohol if they are involved in a fatal accident, and a lengthening of the time that drunk driving convictions remain on a motorist's driving record.

Gov. Harry Hughes also announced this week that the state has received a $400,000 federal grant to pay overtime to state and local police who patrol the highways where drunk driving is most widespread.

With today's change in the law, Maryland becomes the third state to raise the drinking age to 21 after having lowered it to 18 in the early 1970s. Four other states have raised it from 18 to 19 in response to studies showing a correlation between drunk driving and teen-age drinking.

Although Hughes and other lawmakers hailed the new drinking age as a necessary step to curbing drunk drivers, critics charged that the law will accomplish nothing other than impressing voters in an election year.

One of the governor's early polls, released during the winter, showed that a majority of the electorate would support candidates who approve a higher drinking age.