Somatostatin-like immunoreactivity was demonstrated in the external musculature, which includes the myenteric nerve plexus, and in the mucosa-submucosa, which includes the submucous plexus, of the guinea pig ileum using radioimmunoassay. Extrinsic denervation for 22 to 24 days decreased somatostatin concentrations by one third in the external muscle, but no change occurred in the mucosa-submucosa. Somatostatin was demonstrated immunohistochemically in nerve cell bodies and axons of the myenteric and submucous plexuses. Varicose immunoreactive axons were closely related to nerve cell bodies in both plexuses. It is suggested that the somatostatin-containing neurons are interneurons within the enteric nervous system.