These issues are relevant to Mexican heritage youthmany from immigrant familiesin particular ways due to the acculturation process and other contextual factors. The study utilized growth curve modeling to investigate the trajectory of recent substance use (alcohol, cigarettes, marijuana, inhalants) among Mexican heritage students (N=1,670) participating in the keepin' it REAL drug prevention program at different developmental periods: the elementary school (fifth grade), middle school (seventh grade), or both. The findings provide no evidence that intervening only in elementary school was effective in altering substance use trajectories from fifth to eighth grade, either for licit nor illicit substances. Implementing keepin' it REAL in middle school alone altered the trajectories of use of all four substances for Mexican heritage youth. A double dose of prevention, in elementary and middle school proved to be equally as effective as intervening in seventh grade only, and only for marijuana and inhalants. The decrease in use of marijuana and inhalants among students in the seventh-grade-only or the fifth- and seventh-grade interventions occurred just after students received the curriculum intervention in seventh grade. These results are interpreted from an ecodevelopmental and culturally specific perspective and recommendations for prevention and future research are discussed. (Published Abstract) 60 references