Former researches on the contribution of organized sport to antisocial and pro-social behavior of the young have yielded discordant and contradictory results. However, in the majority of researches, it was determined that active practice of sport has a different effect on psycho-social development of male and female participants. We have undertaken the research with the aim of examining whether there is a connection between the participation of the pupils of secondary school age in organized sport activities and their pro-social value orientation and aggressiveness, and, if there is, whether there are gender differences in that connection. The sample consisted of 300 pupils of secondary school (Mathematical High School in Belgrade), out of which 150 (49 female and 101 male) regularly practice sport for more than two years, and 150 pupils (84 male and 66 female) do not practice organized sport. The indicators of practicing sport are: the length of practicing sport activities, regularity and intensity of training, participation in contests and the kind of sport (individual or team sport). The following variables of pro-social orientation were examined: readiness for cooperation, caring for other people, social responsibility and empathy. Aggressiveness was examined through aggressive tendencies towards others. All variables were examined by the application of appropriate questionnaires and evaluation scales which were answered by the pupils themselves. The results have shown significant (although low), positive correlations between practicing sport and pro-social values, as well as the negative (low) correlation between practicing sport and aggression, for both genders of students. Statistically significant differences were found between boys and girls regarding pro-social orientation (higher level in girls), as well as aggressiveness (higher levels in boys). Significant correlations were lower and more numerous in male than in female respondents.