A number of Prince George's County teen-agers in a Riverdale neighborhood located next to the Baltimore-Washington Parkway were eager to explain yesterday why they throw rocks at passing cars.

"It's just something to do," explained one shaggy-haired 14-year-old, who admitted he and others had hurled stones at speeding cars heading down the parkway. "We usually try to see how many cars we can hit and how many windows we can break. It is usually the buses and trucks that we throw at because we're afraid of the cars will stop and we'll get caught."

When asked about the danger to drivers and passengers, he responded: "Oh, the people . . . we don't really think about them."

It was from this neighborhood, called East Pines, that rocks or some other objects were hurled at passing cars Friday night prompting U.S. Park police to clsoe a section of the parkway.

No one was injured, but the incident - which occured between 6:30 and 6:45 p.m. - frightened at least five motorists and had U.S. Park Police at first hunting for a sniper. Police later said they could not determine the nature of the missiles that struck the cars.

The incidents occurred within about a mile of the location where a Grey-hound bus window was shattered by a stone, in September, 1976, and a passenger was seriously injured. At least 15 other rock-throwing incidents occurred in that same area last year.

Motorcycles, trucks and old cars are scattered through this section of the East Pines neighborhood where homes range between $30,000 and $35,000. Neighbors say many of the homes are headed by a single working parent who often does not have time to supervise their teen-agers.

A number of the teen-agers who talked to a reporter yesterday either had quite school or admitted that they frequently cut classes.

Although none of the teenagers yesterday amitted throwing stones at cars Friday night, one parent said she called police after her son told her he saw two of them standing in a drainage ditch at 6:30 p.m. hurling stones at cars.

Rock-throwing on the parkway has become a sort of tradition in this East Pines neighborhood, according to the teen-agers. They say it is part of a stage in their development.

"It was something I did. I don't know why, I guess it was the excitement of it," said another 24-year-old.

He said he used to throw rocks when he was 12 years old but he quit after he almost got caught.

The mother of one of these teenagers who admitted he had thrown rocks said she tried to keep track of her son, but added: "there isn't much I can do."

"The teen-agers are running wild in this neighborhood." The woman said her home and the homes of neighbors have been the larget of vandalism and burglaries by these teen-agers. "They came into my home and ransacked it without stealing anything." she said.

"These teen-agers also held a "pot party" in my grandfather's shed a few blocks away and burned it down."

"They are usually stoned out of their minds," said one teen-ager who is employed as a construction worker. The problem exists because "there is nothing for those kids to do." said a companion.

He and other teen-agers complain that there is no recreation center for them to use, but at the same time, admits that the nearby playground has been constantly damaged by teen-agers.

About the rock throwing, the youth said: "Everybody in the neighborhood is crazy enough to do it." It usually occurs shortly after dark when bored teen-agers get drunk and decide to do something crazy," he said.

Another teen-ager, who said he used to "get high" and throw stones at cars, told a reporter that he and his friends would stand in a drainage ditch that goes under the Parkway and hurl stones at passing cars.

This was our gathering place until the police started coming by and hassling us," said the youth, who showed a reporter a cut on his knee which he said he suffered as he tried recently to crawl through the drain pipe under the parkway.

"We used to take logs out on the parkway and watch cars run over them," added the 12 year-old youth as a grin crossed his freckled face.