The effects of electrical foot shock on the activity of the ascending dopaminergic neurons were estimated in the rat by measuring the changes in DOPAC and DA levels in discrete brain areas. DOPAC and DA levels were estimated with a radioenzymatic method in microdiscs of tissues punched out from serial frontal sections of the brain. A marked rise in the ratio of DOPAC/DA levels resulting from an increase of DOPAC and a decrease of DA levels was found in the cerebral frontal cortex at the end of a 20 min stress. The effect was less pronounced in stress of shorter duration from 3 to 10 min and was only related to a reduction of DA levels. Using the DOPAC/DA ratio as an index of the activity of the neurons, the mesocortical dopaminergic neurons were found to be selectively activated under stress since this ratio was increased in the frontal and cingular cortices but not in limbic structures such as the septum, the amygdala and the nucleus accumbens or in the striatum. Finally, pretreatment of the rats with diazepam (5 mg/kg i.p.) or chlordiazepoxide (10 mg/kg i.p.) prevented the increase in the DOPAC/DA ratio in the frontal cerebral cortex of rats submitted to the 20 min stress.