Eysenck's hypothesis of higher cortical arousal in introverts was examined using regional cerebral blood flow measurement in 37 healthy subjects . The measurement was made at rest, using the133Xe-inhalation method. Estimates of gray matter flow were obtained for 32 brain regions. There was no significant evidence of personality differences in general arousal, as measured by the mean flow level, averaged over all regions. There were, however, regional differences. An overall test of the blood flow distribution indicated different patterns of activity in introverts and extraverts. Follow-up tests attributed this to higher flow in the temporal lobes for introverts than for extraverts. Higher temporal lobe activity in introverts may be interpreted as increased functional connections between cortex and the limbic system.