The mechanisms underlying the suppressant effects of naltrexone (NTX) on ad libitum alcohol drinking in a bar/restaurant setting were investigated in heavy beer drinkers. Fifty-one male and female heavy drinkers (mean age = 22) received 50 mg of NTX or placebo (PBO), p.o., on two separate occasions in a randomized, double-blind crossover protocol. After 7 days of taking medication, subjects were provided with the opportunity to consume beer ad libitum during two, 90-min test sessions that were held 1 to 2 weeks apart. Blood samples were collected on test days to ensure medication compliance and to measure blood levels of NTX and the active beta-naltrexol. Less beer was consumed during NTX treatment. NTX decreased urges to consume alcohol. NTX-treated subjects also took significantly longer to finish each glass of beer and were more likely to terminate beer drinking early. Self-report stimulation and ratings of positive mood states were lower during NTX treatment. Negative side effects of NTX, such as nausea and headache, were reported more frequently with NTX. Not all of the subjects decreased their beer intake on NTX, and some subjects drank more beer. Nonresponders to NTX were not related to blood levels of the active metabolite beta-naltrexol or to a family history of alcoholism. Overall, the results of this study suggest that NTX affects a number of the components of alcohol drinking sequence, including lowering cravings, decreasing the positive reinforcing effects of alcohol, and increasing headache and nausea, each of which may contribute to reducing alcohol intake.