Participants in this 9-week, double-blind, placebo-controlled study will be randomly assigned to receive either selegiline (10 mg/day in two 5 mg pills) or placebo. Study visits will occur twice each week. At each study visit, participants will complete self-reports and urine toxicology tests. Throughout the study, participants will receive weekly individual counseling. The counseling sessions will last 15 minutes. Reduction in marijuana use as well as the number of weeks of consecutive marijuana abstinence will be evaluated.