The influences of gender and sexual maturation on the peak ˙VO2 of 12-yr olds were examined. Subjects were 106 boys and 106 girls, ages 12.2± 0.4 yr. The sexual maturity of 93 boys and 83 girls was classified according to Tanner's indices of pubic hair. No significant gender differences(P > 0.05) were detected in age, stature, or hemoglobin concentration. Peak ˙VO2 was determined on a treadmill and boys' peak ˙VO2 was significantly higher (P < 0.01) than girls' whether expressed in L·min-1 (2.10 ± 0.34 vs 1.92± 0.28) or mL·kg-1·min-1 (52 ± 6 vs 44 ± 5). With body mass controlled for using log-linear ANCOVA, the gender difference decreased from 18.2 to 17.1% but remained significant(P < 0.01). For peak ˙VO2 (L·min-1) ANOVA revealed no significant interaction (P > 0.05) but significant (P < 0.01) main effects for both gender and maturation. For peak ˙VO2 in ratio with body mass(mL·kg-1·min-1), ANOVA detected no significant interaction (P > 0.05) or significant main effect (P> 0.05) for maturation although the main effect for gender was significant(P < 0.01). Analysis of peak ˙VO2 with body mass controlled for using log-linear ANCOVA revealed no significant interaction(P > 0.05) but main effects (P < 0.01) for both gender and maturation. Thus, gender differences, which are not simply explained by differences in body size or hemoglobin concentration, have been demonstrated in the peak ˙VO2 of 12-yr olds. A log-linear scaling model has identified in both boys and girls a significant influence of maturation on peak ˙VO2 independent of body mass. This effect may have been masked in previous studies by the inappropriate use of peak˙VO2 in ratio with body mass.