The purpose of this study is to examine how the assertive-aggresive behavior is maintained, increases and changes its response and victim in the interactions of nursery school children. It is predicted that the consequences of the assertive-aggresive behavior stipplied by the peer group work as positive or negative reinforcers and condition the assertive-aggressive behavior. In order to confirm this predlction, the two following hypotheses are tested. 1. When an assertive behavior is followed by a positive reinforcer, on the next occasion in which the same aggressor is involved he would select the same aggressive response and the same victim. On the other hand when his behavior is follwed by a negative reinforcer, he would change efither the aggressive response or the victimor both.