Regional brain metabolism was determined by positron emission tomography for normal subjects while they performed auditory discrimination, received somatosensory stimulation, or rested. Higher metabolic rates were found in regions of the right middle prefrontal cortex and lower metabolic rates were found in the anterior cingulate and superior posterior parietal cortices of subjects performing auditory discrimination. Furthermore, a direct relationship was observed between metabolic rates in the middle prefrontal cortex and the accuracy of a subject's auditory discrimination, suggesting that this brain region is an important determinant of sustained attention. The somatosensory condition, i.e., receiving 35 min of “meaningless” intermittent electric shock, a condition that has been previously used to study frontal cortex activation in psychiatric disorders, was not associated with generalized activation of the frontal cortex. The higher metabolic rates in the inferior prefrontal cortex and in regions of the temporal cortex observed with shock may relate to habituation, extinction, or inhibition of response.