Interactions between striatal dopamine (DA) and neurotensin (NT) have been suggested by anatomical, behavioral, and biochemical studies. Nigrostriatal DA neurons, in contrast to mesocorticolimbic DA neurons, do not appear to contain NT. Thus, distinct neuronal elements subserve interactions between DA and NT within the striatum. We have previously demonstrated that reserpine-induced depletion of striatal DA is accompanied by a dose- and time-dependent increase in striatal NT concentrations. In order to further characterize the effects of reserpine and to define the mechanism by which reserpine acts to increase striatal NT concentrations, we have used immunohistochemical and biochemical approaches. Immunohistochemical examination of rats pretreated with reserpine revealed marked increases in the density of NT-like immunoreactive (NT-li) perikarya and fibers, and the development of NT-li patches. Pretreatment with reserpine had no apparent effect on NT synthesis, as assessed by examination of cycloheximide-induced inhibition of protein synthesis. However, reserpine administration resulted in a significant decrease in the release of both DA and NT into the striatal extracellular fluid, as measured by in vivo microdialysis. These data suggest that the increase in striatal NT concentrations observed after reserpine treatment results from decreased release, rather than increased synthesis of the peptide.