Previous research has found a relation between substance use and academic achievement and motivation, although the direction of the causal relation is unclear. Whereas numerous researchers have found that substance use is predictive of graduation from high school, others have found that poor academic achievement and low academic aspirations have a significant influence on substance use onset. In this study data from a 3-year longitudinal study of adolescent substance use were used to clarify the direction of influence between the adolescent's academic achievement and motivation and substance use. Adolescents (N=464), ages 12-16, from grades 6 through 11, and their parent(s) completed parallel questionnaires measuring marijuana, cigarette, and alcohol use; achievement motivation; and academic achievement. The results suggest that the relation between substance use and achievement and motivation is bidirectional. Adolescents with lower motivation tended to initiate the use of marijuana and initiation of cigarette use tended to decrease the adolescent's achievement and motivation. For 12-year-olds, only the path between substance use and academic achievement was significant. (ABL)