ABSTRACTPsychophysiological tests of the Eysenck hypothesis that introverts are more arousal prone than extraverts have yielded somewhat mixed results. In an attempt to clarify and extend previous findings, the present study manipulated arousal level, using caffeine. Extreme groups of 48 extraverts and 48 introverts were used, and each subject was randomly assigned to a low, medium, or high dosage of caffeine or a placebo. Each subject heard a series of tones until criterion electrodermal habituation occurred. This was followed by dishabituation, overhabituation, and a further dishabituation procedure. Increasing caffeine dosages produced a linear increment in tonic levels in extraverts, but had little effect on SCL in introverts. Higher caffeine dosages also increased phasic response amplitudes in extraverts but decreased amplitudes in introverts. Although somewhat complex, results were partially supportive of the Eysenck hypothesis.