A teen-age robbery suspect was injured last night when he was struck by an Arlington police cruiser after pointing a gun at a police officer who then fired at him and missed, according to county police.

The incident followed a car chase and tied up traffic on Jefferson Davis Highway, police said.

The youth was taken to the National Orthopaedic and Rehabilitation Hospital, where he was treated for minor injuries. A second youth was seized by police in an auto they stopped in the 1400 block of Jefferson Davis Highway, and an intensive search was going on early today in the Pentagon City area for a third suspect.

The chase followed the holdup of a woman in the parking lot of the Ramada Inn on Kenmore Avenue in Alexandria by three persons who fled in an automobile. Arlington police said they joined the chase after receiving a radio description from Alexandria officials.

Arlington police gave this account: The chase ended about 10:15 p.m. when an Arlington officer pulled his car into the path of the suspects' vehicle just south of the 14th Street Bridge approaches.

The cars halted inches apart. A youth jumped from the passenger side of the suspects' car, and a police officer from the other car. The youth pointed a gun at the officer, and the officer fired his gun and missed.

Then the youth darted to his right as a second police car, which was arriving on the scene, swerved from behind the suspects' vehicle and struck the youth.