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This study examined the impact of body mass and body image on autonomous motivation for exercise among adolescents. It was predicted that body mass and body size discrepancies would be curvilinearly related to relative autonomy because, from a self-determination theory perspective, being or perceiving that one is under- or over-sized would be experienced as pressure to conform to culturally transmitted standards of an ideal physique, undermining one's sense of autonomy.Cross-sectional comparative study.Fifty males (mean age 16.90) and 48 females (mean age 16.88) completed measures of relative autonomy for exercise, discrepancies between perceived and ideal body size, body mass index and physical activity.Hierarchical polynomial regression analyses showed that among males relative autonomy was predicted by both body mass and body size discrepancies. The relationships took an inverted-u form: autonomy was at its maximum when body mass index was around 18.50 and when body size discrepancies were zero. Among females, relative autonomy was predicted by body size discrepancies alone and the relationship was r-shaped: autonomy increased as body size discrepancies became less negative, reaching a maximum and leveling off when the discrepancy was +1.The gender difference in the effect of body mass and perceived body size discrepancies on autonomous motivation for exercise could be explained by different socio-cultural expectations for males and females in Western societies. For females the cultural norm is a thin and toned physique whereas for males it is a muscular mesomorphic build that is neither thin nor fat.