This study evaluated the effects of a school-based preventive intervention (Botvin, G. J. 1996, 2000) on growth trajectories of substance initiation (alcohol, tobacco, and marijuana), expectancies, and refusal intentions. A rural midwestern sample (N=847) provided three waves of data from middle school students. Growth curve analyses demonstrated that the intervention significantly slowed the rate of increase in substance initiation and significantly slowed the rate of decrease in refusal intentions. The intervention also slowed the rate of decrease in negative outcome expectancies, although the significance level was only marginal. A multiple group comparison showed that the impact of initial levels of substance initiation on growth trajectories of refusal intentions differed between conditions, suggesting that the intervention decreased the effect of early substance initiation on the rate of change over time for refusal intentions. Gender differences also were found, although the intervention was effective in slowing the rate of increase in initiation for both genders.