The neurophysiological correlate of high levels of activation has been considered to be a state of high cortical facilitation.' On the basis of this hypothesis, it is reasonable to expect that the output from an effector of a highly aroused organism would be greater than that of a lesser aroused organism, when both are equally stimulated. It has been suggested by the writer2 that the personality dimension introversion-extraversion, as measured by Heron's scale,3 is related to degree of arousal. If introverts are in general more highly aroused than extraverts, then, assuming that arousal is synonymous with a state of high cortical facilitation, it follows that the output of an effector of an introvert should be greater than that if an extravert when both are equally stimulated. Thus, a few drops of lemon juice applied to the tongue of an introvert should, in a given interval of time, produce a greater output of saliva than from an extravert. Introverts should also be more susceptible to painful stimuli and less susceptible to sensory deprivation-both of which have recently been demonstrated.4