The functional effect of neurotensin on the kinetics of dopamine (DA) release in the substantia nigra of the freely moving rat was investigated. After guide tubes for push-pull perfusion were implanted stereotaxically just above the substantia nigra, endogenous stores of DA in this structure were labelled by micro-injection of 0.02-0.05 microCi of [14C]-DA. Then an artificial cerebrospinal fluid (CSF) was perfused within the site at a rate of 20 microliters/min at successive 5 min intervals. Neurotensin added to the CSF perfusate in concentrations of 0.05-0.1 microgram/microliter evoked an immediate, Ca++ dependent release of DA from sites directly within the substantia nigra or a delayed efflux when the peptide was perfused at the edge of this structure. Neurotensin failed to affect the pattern of release of this monoamine at sites which were not within the substantia nigra. Further, the body temperature of the rat also was not altered by neurotensin at any of the sites of perfusions. A relatively inactive analogue of the peptide, [D-Arg]9 neurotensin, was essentially without effect on DA activity. In double isotope experiments in which the substantia nigra of the rat was labelled with both [3H]-5-HT and [14C]-DA, the perfusion with neurotensin failed to affect 5-HT efflux while the release of DA was enhanced. Chromatographic analysis of the metabolites of DA in samples of push-pull perfusates revealed that neurotensin enhanced significantly the level of DOPAC and HVA. Overall, these results demonstrate that in the unrestrained rat neurotensin acts selectively within the substantia nigra to alter the presynaptic, Ca++ dependent release of DA.(ABSTRACT TRUNCATED AT 250 WORDS)