Two years ago, after the Maryland General Assembly prohibited public drinking, the crowds of teen-agers who frequented Montgomery County shopping centers to drink and socialize thinned out. The word got around that what they were doing was against the law and drunk offenders would be arrested.

But another message among the youths: The police weren't going to bother to enforce the law. The teen-agers went back to the shopping centers and resumed their outdoor socializing and drinking on street corners, culde-sacs and parking lots in such communities as Rockville, North Bethesda, Aspen Hill and Damascus.

The absence of possible fines against teen-agers rendered the law a paper tiger, according to law enforcement officials, since the worst punishment usually consists of referring the problem to the offenders' parents.

To resolve the difficulties of enforcing the current law, which applies statewide, State Sen. Charles W. Gilchrist (D-Montgomery) asked a state Senate committee yesterday to approve a bill that would provide for fines up to $25 for teen-agers caught drinking in public.

It would "put teeth in the law," said Gilchrist, a Democrat who is seeking his party's nomination for Montgomery County executive.

"No matter how hard a parent tries, a child is going to do what he wants to," Virginia Wellett, a Montgomery County parent and activist testified. "They are drinking under 14 years old today and smoking pot, too," "Youths, she said, should fine ways to earn money to pay off any drinking fines.

The current law, she complained "is just a slap on the wrist with a wet noodle." She declared that Gilchrist's bill is "the greatest thing since the Pill." That assertion drew guffaws from members of the Senate Committee on Judiciary Proceedings.

The teen-age public drinking problem in Montgomery before the current law was passed in 1976 was described as immense. In 1975 two teen-agers who were part of the shopping center and outdoor drinking crowd were murdered.

In 1976, after the law went into effect, 407 juveniles were arrested for public drinking in Montgomery County. The following year only 184 were arrested. The reason for the decrease was not a drastic decline in the teen-age drinking problem, according to Lt. Paul Whitling of the county police youth division.

"It's police indifference" to an unforceable law, he testified.

On the other hand, since adults could be fined under law, instances of public drinking of 18 to 23 year-olds have declined, he said.

The Suburban Maryland Alcohol Council reported recently that 6,583 Montgomery youths were "problem drinkers." In addition, a council study showed that 48 percent of county teen-age males interviewed said they drank once a week and 57.5 percent admitted driving while drunk.

While instances of insults and assaults drinking teen-agers in large shopping centers and malls have declined, such problems persist in outer reachers of the county where there are fewer police, authorities said.

In Rockville, some street corners and cul-de-sacs still collect more than 200 teen-agers on Friday nights in the summer, according to Capt. Robert Ash, assistant police chief.

"Most people in the youth field have come around to the idea that a fine would be therapeutic," said Charles L. Short, deputy coordinator of youth services for Montgomery county.

Sen. J. Joseph Curran Jr. (D-Baltimore), committee chairman, said he expects Gilchrist's bill to pass his committee. However, a related Gilchrist proposal, which would fine youths or their parents up to $500 for any juvenile offences might not survive his committee, Curan said.