We investigated the effects of competitive and cooperative games on aggressive and cooperative behaviors of 70 children (4 to 5 years old) from four classes in three preschools. The experimental design included both multiple baseline and reversal components. Behaviors were measured during game conditions and in subsequent free-play periods. Results showed that cooperative behavior increased and aggression decreased during cooperative games; conversely, competitive games were followed by increases in aggressive behavior and decreases in cooperative behavior. Similar effects were also found during free-play periods.