This study examined the relative ways in which muscularity and thinness concerns longitudinally influence adolescents’ intentions to use doping substances. It was hypothesized that muscularity and thinness exert their effects on doping intentions by promoting endorsement of positive attitudes toward doping use in male and female adolescents and across different levels of sport involvement. To test this hypothesis, nearly 900 high school adolescents provided questionnaire data on two separate occasions during an academic year. On average, boys, as well as boys and girls who practice some sport, had relatively strong concerns about muscularity, whereas girls showed relatively strong thinness concerns. Boys also expressed more positive attitudes about doping than did girls. Structural equation modeling showed that muscularity and thinness have direct effects on adolescents’ intentions to engage in doping and that muscularity, but not thinness, partly exerts its effects through the endorsement of positive attitudes toward doping.