Measures of regional cerebral blood flow (rCBF) indicate patterns of brain activity with great potential usefulness for exploring the biological bases of personality traits. We have examined the distribution of cortical rCBF induced by spontaneous mental activity in the resting state, and correlated regional activation with questionnaire measures of personality traits. Our main interest has been in the dimension of extraversion-/introversion. Over a series of three studies (total N=75), results have consistently shown a relationship between high temporal lobe activity and introversion. Task-induced mental activation has tended to diminish the personality differences found at rest.