Intrinsic nitric oxide synthase (NOS)-containing nerve cells and fibers were studied in the wall of the pylorus of cat at the ultrastructural level using ABC immunocytochemistry. Large numbers of NOS immunoreactive (IR) nerve cell bodies were observed in the myenteric and in the submucous plexuses, and few in the tunica propria mucosa. The NOS IR nerve fibers were most abundant in the inner circular muscle layer and in the tunica mucosa. They were found in very close vicinity to the smooth muscle cells of the inner circular muscular layer as well as to the blood vessels and the epithelial lining. The gap between the NOS IR nerve fibers and the membrane of the target cells was 20 to 250nm. Apparent synaptic contacts were observed between the IR nerve fibers and unlabelled nerve processes and other non IR nerve cell body. It is confirmed that NO might influence smooth muscle cell activity, regulate blood flow and modulate the function of the epithelial cells. Our ultrastructural study suggested that some of the NOS containing neurons belong to the intrinsic interneurons and have a regulatory effect on other intrinsic nerve elements involved in local neuronal reflexes.