Overflow of dopamine has been measured in the striatum of anesthetized rats with 60-Hz, 300-microA electrical stimulations of the medial forebrain bundle. Electrodes were placed in the region of the caudate-putamen or the nucleus accumbens. The elliptical electrodes were fabricated from carbon fibers and had a major radius of 35 microns. Overflow was detected with fast-scan voltammetry repeated at 100-ms intervals. In both brain regions the detected substance had the voltammetric properties of dopamine. Measurements were made at different vertical electrode positions, varied by 100-microns intervals. Significant differences in stimulated overflow were observed between each position in both brain regions. The dimensions over which overflow was observed compare to that of previous descriptions of the patch and matrix topology of the striatum. When the electrode was moved in 10-microns intervals the variance was considerably less. The observed rate of overflow during stimulation exactly correlated with the maximal amount of dopamine observed during a stimulation. In addition, the overflow rate and the disappearance rate also correlated. This suggests that in both brain regions uptake and release sites are anatomically close to one another. The major difference between overflow curves measured in the two different regions was the appearance of an apparent mass transfer barrier to the electrode in the caudate-putamen.