Convergent electroencephalographic, performance, and psychometric measures of arousal were used to test predictions based on H. J. Eysenck's (1967) theory. Twelve introverts and 12 extraverts had their brainstem auditory evoked responses (BAERs) monitored under 3 levels of caffeine administration (0.0, 1.5, and 3.0 mg/kg) and 3 levels of task demand (resting, simple response time, and complex response time). Results revealed a decrease in the latency of Wave V of the BAER as a function of personality, with introverts evidencing significantly shorter latency of Wave V as compared with extraverts. Faster conduction time between Waves I-III and I-V were also found in introverts. Caffeine at either the 1.5 or 3.0 mg/kg level was associated with decreased latency of Wave V, compared with the placebo condition. No personality differences in subjective arousal or reaction time performance were found across any of the conditions.