The effects of caffeine on autonomic nervous system (ANS) activity were tested in 20 adult males who were either high or low consumers of caffeine. Subjects received placebo, 3 mg/kg, and 10 mg/kg of caffeine in a counterbalanced order (double-blind) before 3 test sessions 48 h apart. Skin conductance (SC), heart rate, and skin temperature (ST) were recorded during a rest period, a series of non-signal tones, and a simple reaction time task. Caffeine increased resting electrodermal activity (EDA) and increased the SC orienting response to the first non-signal tone, but reduced the increase in tonic EDA due to task performance. ST was reduced by caffeine in both rest and task periods. Increases in nervous/jittery ratings occurred after caffeine ingestion, but task performance was not affected. Low consumers of caffeine showed significantly more ANS responsivity than high consumers under all conditions and did not differ in ANS, behavioral, or subjective effects of caffeine. The acute physiological changes are partly similar to those reported for patients with anxiety disorders, suggesting a possible role of ANS activity in mediating the anxiogenic effects of caffeine. Effects of user status may reflect a predisposing trait, but an effect of chronic caffeine use on ANS sensitivity cannot be ruled out.