In viva voltammetry has been used to measure the release of dopamine evoked by electrical stimulation of the medial forebrain bundle (MFB). Simultaneous measurements have been made with voltammetric-sensing electrodes ipsilateral to the stimulating electrode in the nucelus accumbens and the caudate nucleus of the anesthetized rat. During the stimulation, the species observed in both regions is voltammetrically identical to dopamine. Further evidence for the identity of dopamine is provided by anatomical, physiological, pharmacological, and postmortem data. Postmortem analysis of these brain regions after a single stimulation demonstrates that dopamine levels are unchanged, while dihydroxyphenylacetic acid (DOPAC) levels are increased in both regions. Systemic application of synthesis inhibitors results in a decrease in evoked release for each brain region. Amfonelic acid results in a restoration of stimulated release after synthesis inhibition. Evoked release is affected differently by pargyline in the two brain regions. The evoked release of dopamine is significantly elevated in the nucleus accumbens as a result of pargyline administration, but similar effects are not seen in the caudate nucleus. Tissue levels of dopamine are increased in both brain regions by pargyline, but the increase is significantly greater in the accumbens. Electrolytic lesions of the striatonigral pathway or systemic administration of picrotoxin eliminates the pargyline-induced difference in evoked release of dopamine. Amphetamine causes a reduction in stimulated release in the caudate nucleus with little effect on that observed in the nucleus accumbens. Administration of pargyline prior to amphetamine results in a diminution of release in both brain regions. Taken together, these data indicate that different factors affect regulation of the releasable pool of dopamine in the nucleus accumbens and caudate nucleus.Several major dopaminergic systems exist in the mammalian brain, including the nigrostriatal and mesolimbic systems. It is known that the mesolimbic system plays an important role in the regulation of normal brain function, and this role is distinct from that played by the nigrostriatal dopamine pathway (Mogenson and Yin, 1981;White and Wang, 1982). It has been shown by unit-recording techniques that these two regions respond in different manners to pharmacological stimuli thought to affect dopaminergic neurons (Rebec and Zimmerman, 1980; Reynolds et al., 1981). The behavioral responses elicited by electrical stimulation of these two pathways differ (van der Heyden, 1984). Lesions of the two regions also produce different behavioral responses (Kelly and Moore, 1976; Pycock and Marsden, 1978). Furthermore, anatomical differences have been shown at an ultrastructural level (Bouyer et al., 1984). A rec- Received June 10, 1985; revised Oct. 7, 1985; accepted Oct. 10, 1985. This work was supported by a grant from the National Science Foundation, Neurobiology Section.Correspondence should be addressed to R. M. Wigbtman, Department...