 Laboratory animal studies demonstrate that endogenous cannabinoids and opioids are closely inter-related. We have completed a series of studies in marijuana smokers showing that a clinically-utilized dose of naltrexone 50 mg enhanced the reinforcing and subjective effects of orally-administered tetrahydrocannabinol THC, while a low naltrexone dose 12 mg blunted the effects of THC. A better understanding of the effects of a range of naltrexone doses in combination with smoked marijuana has important implications for the following reasons: 1 Alcohol- and opioid-dependent patients receive high doses of naltrexone 50-150 mg, which may increase the abuse liability of marijuana, 2 Low-dose naltrexone blunts THCs intoxicating effects, suggesting potential utility as a treatment medication for marijuana dependence. This study will determine if naltrexone 0, 12, 25, 50, 100 mg administration 45 min prior to marijuana administration 0, 3.9 THC alters marijuanas subjective, cognitive or physiological effects. Marijuana smokers will spend approximately 5hday for a total of 10 days in the outpatient laboratory. Participants will visit the outpatient laboratory 2-3 times per week, with a minimum 48-hr interval between sessions to allow for naltrexone clearance. These data will provide important information regarding the clinical use of naltrexone. 