In vivo voltammetry was used to study the regional distribution of extracellular ascorbate (AA) and 3,4-dihydroxyphenylacetic acid (DOPAC), a major dopamine metabolite, in the striatum of the rat. An electrochemically-modified carbon-fiber electrode, which provides distinct oxidation curves for each of these substances, was lowered in 1-mm increments through one of four striatal regions selected to sample the entire extent of this structure, including the nucleus accumbens. In anteromedial striatum, the level of AA was highest in the most dorsal and ventral aspects and lowest in the middle, whereas DOPAC levels generally showed the opposite pattern. This inverse relationship between AA and DOPAC was not evident in either lateral or posterior areas. To the extent that AA and DOPAC are released from different axon terminals, as mounting evidence suggests, regional differences in the extracellular concentration of these compounds may reflect the different and, in some cases, reciprocal distributions of two neuronal populations.