Electron microscopic immunocytochemistry was used to localize immunoreactivity for nitric oxide synthase (NOS) in whole-mount preparations of myenteric plexus and circular muscle from guinea-pig ileum. NOS immunoreactivity was patchily distributed in myenteric neurons and was not specifically associated with any subcellular organelle or with the plasma membrane. This localization leaves unanswered the question of how nitric oxide is stored and released. NOS immunoreactive fibres in the circular muscle were found closer than 100 nm to muscle cells. NOS immunoreactive nerve fibres made synaptic contacts with NOS immunoreactive and non-immunoreactive enteric neurons. These results indicate that nitric oxide may regulate the activity of both myenteric neurons and smooth muscle.