There is a good deal of evidence, particularly from electrodermal and electrocortical recording procedures, that introverts exhibit greater reactivity to sensory stimulation than extraverts. There is little evidence that introverts and extraverts differ in base level of arousal in neutral conditions, and there is no clear evidence that their differences in sensitivity to stimulation are determined by differences in attentional state. Faster auditory brainstem evoked response latencies observed for introverts implicate differences in peripheral sensory processes that are not determined by mechanisms in the reticular system as proposed in the arousal hypothesis. There is also evidence that individual differences in the expression of motor activity between introverts and extraverts involve differences in motoneuronal excitability.