In 3 separate testing sessions, 8 male moderate drinkers drank placebo and drinks with small (1.0 ml/kg of body weight) and large (1.5 ml/kg) amounts of alcohol. During each session, blood alcohol concentration (BAC) was measured, various physiological tests were conducted, and each man rated his sensations of sobriety-drunkenness and other psychological parameters. There were 2 patterns of response to similarly rising BAC: 4 men (thresholders) felt that they remained relatively sober prior to an abrupt increase in sensations of intoxication, and 4 (nonthresholders) reported a gradual progression to intoxication. While drinking, nonthresholders showed increased autonomic arousal, accompanied by decreased happiness and sociability. Thresholders showed decreased autonomic arousal, accompanied by increased happiness and sociability, and a reversal of these trends when they passed their threshold. The thresholders showed a more marked effect of rising BAC on positional nystagmus than the nonthresholders.