The effect of a high-opportunity (HO) versus a low-opportunity (LO) play environment on the aggression of preschool children was investigated, hypothesizing that the LO play environment will elicit significantly more aggression than the HO play environment. The two environments were presented randomly to one group of 10 4-year-olds, five of each sex, on 12 consecutive 15-minute play sessions, with the stipulation that each environment be presented six times. There was no pre-experimental familiarization period due to information, later proven incorrect, that the subjects were adequately familiar with all the play apparatus. Results indicate that the high-level aggression in the first two LO sessions is solely responsible for the significant difference between the aggression in the LO versus the HO environments. Anecdotal records indicate that this high-level aggression can be attributed to a novel piece of equipment, a rocker, resulting in a recommendation for further study of the relation between novelty and aggression. The discussion identifies some factors relevant in the aggression education of young children. The design of similar studies using subjects who would presumably exhibit high-level aggression in high-opportunity environments is recommended. (Author/NH)