Using a social constructionist perspective, we investigated the flexibility of early adolescents' (N = 80, 40 boys, 40 girls; M age = 13.14; SD = 0.65) masculinity and femininity as a function of the interpersonal context (same- or other-sex partner) and situational demands (co-operation or competition). Participants played a block-building game with a girl and a boy peer and were instructed either to play the game co-operatively or competitively. Boys' and girls' femininity scores were greater when working with a girl peer rather than a boy peer. Stereotypical gender differences in masculinity were most apparent when instructions emphasized co-operation. These findings provide empirical support for a social constructionist theory of gender development by demonstrating how the proximal situation changes adolescents' views of gendered aspects of their identity. A conceptualization of masculinity and femininity as flexible states is discussed as supplementing the idea that masculinity and femininity are enduring personality traits. Implications of the study for understanding how social situations maximize or minimize gender differences and similarities are discussed.