Nerve fibres and cell bodies showing immunoreactivity for somatostatin were found in the myenteric and submucous plexuses of the guinea-pig small intestine. Both nerve fibres and endocrine cells of the mucosa were reactive. Acid extracts of the myenteric and submucous plexus were prepared. Activity in the extracts was separated by high pressure liquid chromatography, using reverse phase and molecular sieve columns. In both cases, the eluted peaks corresponded in retention time to authentic somatostatin. Derivatives of the reactive material in extracts and of authentic somatostatin were made by oxidation using H2O2. These derivatives had different retention times than the original material on reverse phase chromatography, but there was still correspondence of the peaks from oxidized extracts and oxidized authentic somatostatin. It is concluded that somatostatin-like immunoreactivity in intestinal nerves is due to the authentic peptide.