Slices obtained from the deeper layers of the rat dorsal frontal, parietal and occipital brain cortex were incubated in vitro with 6.25 X 10-7 M [3H]dopamine (DA), and subsequently superfused and electrically stimulated, while held on quick transfer electrodes, and changes in the efflux of 3H and of the individual amines measured. The separation of the amines, with quantitative recoveries, was performed by chromatography on cation-exchange resins eluted sequentially with water, 1 N HCL AND 6 M urea i 1 N HCl. When no drugs were used, the prestimulation efflux was entirely formed by deaminated metabolites, while following stimulation there was an increase in the efflux of deaminated metabolites, and considerable amounts of [3H]-noradrenaline (NA) now appeared. No DA was present in the pre- or poststimulation medium. Similar results were obtained in all the regions studied. When the slices were incubated with 10-5 M desmethylimipramine (DMI), 10-4 Mnialamide and 10-4 M tropolone, before and during incubation with [3H]DA, it was observed that, prior to stimulation, the efflux was composed of deaminated metabolites, DA and 3-methoxytyramine (MTA), and following the electrical stimulus there was an increased release of DA, NA and deaminated compounds (in order of decreasing release), while no change in that of MTA was evident. The stimulus-induced release of DA was greatest from frontal slices, intermediate from parietal, and lowest from occipital ones. DMI-resistant uptake of [3H]DA also diminished when passing from frontal to occipital. These findings are interpreted as due to the presence of dopaminergic axon terminals in all the regions studied, but with a density that diminishes in a rostrocaudal direction.