A counterbalanced within-subjects design was employed to assess the acute effects of alcohol ingestion on thirst and fluid intake and to relate these effects to the dehydration known to result from acute ingestion of alcohol. Fourteen male nonalcoholic social drinkers between the ages of 18 and 30 consumed 0.8 gm of alcohol/kg body weight during one experimental session and a placebo (0.05 gm/kg) of equal volume during the other. During the 90-minute drinking period and for one hour both before and afterward, urine specimens were collected. After the drinking period, blood alcohol levels and unobstrusive measures of ad lib drinking were recorded. In addition, subjects rated their degree of thirst three times during each session. Analyses of variance indicated that alcohol significantly increased both fluid intake and urine output, and decreased urine specific gravity. Moreover, thirst ratings were also significantly greater after alcohol than after placebo and fluid intake after alcohol was significantly correlated with prior measures of fluid balance, mean urine specific gravity, peak blood alcohol level and thirst. The implications of these results for the loss of control in alcoholism are discussed and a new theoretical account of the phenomenon is proposed.