Anatomical, biochemical and behavioral data suggest functional interactions between dopamine and neurotensin in regions of the brain receiving a co-existent and/or distinct innervation by these two transmitters. We therefore measured and compared the effects of iontophoretically applied dopamine and neurotensin in the prefrontal and anterior cingulate cortex (co-existent innervation) vs the nucleus accumbens and neostriatum (distinct innervation) of urethane-anesthetized rats. In every region, the firing rate of most spontaneously active neurons was depressed by dopamine. Neurotensin had no effect on the same cells, except for a few nucleus accumbens units which were inhibited by the peptide. When dopamine and neurotensin were concomitantly applied, the magnitude of maximal inhibitions induced by dopamine was modified in the majority of neurons tested. A significant decrease in dopamine inhibition was observed in 100% of anterior cingulate, 74% of prefrontal cortex and 48% of accumbens units. On the contrary, in neostriatum, dopamine inhibition was significantly increased in 60% of the units tested. In every region, the remaining neurons showed less than 30% changes in dopamine responsiveness, and were therefore considered unaffected by neurotensin. In the anterior cingulate cortex, inhibitions, respectively, induced by the dopamine D1 agonist, SKF 38393, and the D2 agonist, LY 171555, were also decreased by simultaneous application of neurotensin. Together with currently available data on the cellular localization of neurotensin receptors in rat brain, these results suggest that the modulation of dopamine inhibition by neurotensin may have opposite effects depending on whether the neurotensin receptors are located postsynaptically on target neurons (antagonistic effects) or presynaptically on dopamine terminals (potentiating effects).