Theory and research is described which led to the hypothesis that the choleric (E+N+) and melancholic (E-N+) temperaments are determined by differences in thalamocortical inhibition of brain-stem processes. An opportunity to test this hypothesis was provided by the recent discovery of 4, 7, and 10 Hz response waves confounded in EEG averaged evoked potentials (D. L. Robinson, 1999b). These responses are attributed to the brain-stem, limbic, and thalamocortical arousal systems, respectively, and in the cited reports principal components analysis of data obtained from 93 Ss (mean age 55.1 yrs) confirmed the existence of predicted excitatory and inhibitory relationships. ANOVA was used to test the further prediction of strong inhibition of the 4 Hz system by the 10 Hz system in melancholics (E-N+) and weak inhibition of the 4 Hz system in cholerics (E+N+), with median inhibition predicted for the other temperament types. There was a large and statistically significant difference between the mean scores on the principal component analsysis inhibition factor obtained for the temperament groups, with temperament defined in terms of high and low extraversion and neuroticism scores.