In this investigation, two indicators of high school educational success (grade point average and college plans) were examined to determine their influence on cigarette, alcohol, and illicit drug use during the transition to young adulthood. Panel data were obtained from the 1978-81 Monitoring the Future nationally representative samples of high school seniors. The sample for this analysis included 3,399 White respondents (weighted cases) followed three to four years post-high school. Structural equation models that included background and mediating characteristics were developed and tested using LISREL VII. Consistent with a selection hypothesis, high school GPA had a negative indirect effect on post-high school substance use that operated largely via senior-year substance use. College plans during high school had a similar negative effect on post-high school cigarette use, but consistent with a differential-socialization hypothesis, they had a positive indirect effect on post-high school alcohol use that operated primarily via student and marital status during young adulthood. College plans had no impact on post-high school illicit drug use.