Three major hypothses were tested: (a) sex differences in play behavior are present in 3 year olds, (b) female teachers reinforce feminine behaviors, (c) peers reinforce like-sex peers. Two nursery school classes, comprised only of 3 year olds, were observed on a sex-role behavior checklist inermittently throughout the year. Each class had 18 students, 9 girls and 9 boys. Sex appropriate behaviors were present in both the schools from the beginning of the year and were stable over time. The data showed that teachers consistently reinforced feminine-type behaviors more than masculine behaviors, and reinforced boys but not girls when they performed opposite-sexed behaviors. Peers, on the other hand, reinforced mostly same-sex peers.