Substantial amounts of somatostatin have been demonstrated in the basal ganglia and immunocytochemistry has demonstrated both somatostatin staining neurons and nerve terminals. In order to study possible sources of somatostatin input to the striatum a series of lesions were made. Lesions of the substantia nigra produced with either 6-hydroxydopamine or ibotenic acid and hemisections at the level of the globus pallidus (6.2 mm anterior) had no effect on striatal levels of somatostatin-like immunoreactivity (SLI). Similarly dorsal frontal or hemicortectomies had no effect on SLI in the striatum. Kainic acid injected directly into the striatum at 2 dose levels resulted in a 60% depletion of SLI on the ipsilateral side and a 25% reduction on the contralateral side. These results suggest that about half of the SLI in the striatum is localizable to intrinsic neurons; the source of the remaining SLI is presently unknown but does not appear to arise in the dorsal cortex, thalamus or brainstem.