Theoretical models of alcoholism emphasize the acute reinforcing properties of alcohol as chief determinants of drinking, and animal research suggests adolescents are uniquely sensitive to these effects. Human studies of these phenomena, however, are virtually nonexistent. We used ecological momentary assessment methods to capture adolescents' subjective responses to alcohol in real time in their natural environments. Adolescent participants were 22 problem drinkers, ages 15 to 19 years (M = 18.3, SD = 0.09; 55% female; 55% alcohol dependent). Participants consumed alcohol on 38% of days during a 1-week monitoring period, with an average of 5 drinks per occasion. Momentary data revealed that adolescents experience decreased stimulation and increased sedation and "high" across the ascending limb of the blood alcohol curve. Notably, greater craving predicted higher volumes of subsequent alcohol consumption during the episode, whereas greater "high" attenuated use. To test for developmental differences in these effects, we pooled these data with data from a similarly ascertained sample of 36 adult heavy drinkers, ages 24 to 64 years (M = 38.1, SD = 11.8; 50% female; 61% alcohol dependent). Adolescents were more sensitive to the stimulant effects of alcohol than adults. This study provides novel data on how adolescent problem drinkers experience alcohol in their natural contexts and illustrates how these effects, which appear to differ from adult problem drinkers, confer liability for future drinking.