Youngsters in the Arnhem Zoo chimpanzee colony frequently "bother" adult group-members. They throw sticks and sand at them, hit them from behind, etc. , and dash away immediately. This occurs without apparent instigation and in spite of the fact that they may be punished. Earlier data led to the hypothesis that this so-called quasi-aggressive behaviour (which also occurs in wild chimpanzees and in other species) is a form of social exploration caused by exploratory impulses which functions as a mechanism for youngsters to learn and expand social limits. To test this hypothesis new data were obtained on the same individuals. The results provide firm evidence that quasi-aggressive behaviour is indeed social exploration. Two different functional forms could be distinguished. 1. Uncertainty reduction. This form of social exploration is performed by male and female youngsters alike, especially towards adult females. The behaviour is more likely, the higher the level of response variability that the youngsters experience in their relationship with these females. This is indicated by the fact that there are strong positive correlations between response variability (as measured by the information-theoretical measure of uncertainty H) and frequency of quasi-aggressive behaviour performed. Female reactions tended to be highly variable and included many aggressive and fearful responses. The course and patterning of quasi-aggressive behaviour was strongly influenced not only by the variability in response types but also by the quality of the reaction. Quasi-aggressive behaviour continued more often after aggressive and fearful responses. Aggressive responses in particular resulted in longer bouts of quasi-aggressive behaviour, whereas fearful responses (which were stimulating initially) resulted in shorter bouts. After the behaviour had terminated it was repeated more quickly if the response of the target animal had been aggressive. Consistent ignoring reactions on the part of the target animals resulted in fewer and shorter bouts, which were repeated less quickly. It is argued that the results indicate that this type of quasi-aggressive behaviour is aimed at reducing uncertainty in the first place, whereas a secondary goal of the youngsters is to be able to "control" the (responses of) target animals. 2. Investigating authority. Quasi-aggressive behaviour performed towards adult males, especially by male youngsters, differs considerably from the "reducing uncertainty" type directed towards adult females. Instigating factors are: high adult male dominance rank and display of power by these high ranking males (viz. when they are 'bluffing'). The behaviour of the youngsters performing this type of quasi-aggressive behaviour is highly ambivalent: they show a clear intention to withdraw, their behaviour is often accompanied by 'submissive greeting', they show signs of fear relatively quickly, in spite of the fact that the males seldom react aggressively. The reaction of the males varies very little: on the whole (over 70% of the times) they ignored the youngsters. In the main, bouts against males lasted for a shorter time and were repeated less quickly (in spite of the fact that the males did not receive less quasi-aggressive behaviour than did the females). On the whole quasi-aggressive behaviour directed towards males was less associated with the responses of the targets than that directed toward females. The "investigating authority" type of quasi-aggressive behaviour is related less to uncertainty and more to male dominance rank. It functions as a means for youngsters to learn about power relationships and about the constituents of "dominant behaviour". Both forms of social exploration test certain characteristics of the social environment of youngsters, viz. the nature of relationships of which the youngsters themselves form a part. It is argued that this effect is not achieved by any other behaviour. It is just as adaptive for youngsters to explore their social environment as it is for them to explore their physical environment: through exploration youngsters gain vital knowledge for a proper functioning in their surroundings.