The ultrastructure and immunocytochemistry of the canine distal pyloric muscle loop, the pyloric sphincter, were studied. Cells in this muscle were connected by gap junctions, fewer than in the antrum or corpus. The sphincter had a dense innervation and a sparse population of interstitial cells of Cajal. Most such cells were of the circular muscle type but a few were of the type in the myenteric plexus. Nerves were sometimes associated with interstitial cell profiles, but most nerves were neither close to nor associated with interstitial cells nor close to smooth muscle cells. Nerve profiles were characterized by an unusually high proportion of varicosities with a majority or a high proportion of large granular vesicles. Many of these were shown to contain material immunoreactive for vasoactive intestinal polypeptide (VIP) and some had substance P (SP) immunoreactive material. All were presumed to be peptidergic. VIP was present in a higher concentration in this muscle than in adjacent antral or duodenal circular muscle. Interstitial cells of Cajal made gap junctions to smooth muscle and to one another and might provide myogenic pacemaking activity for this muscle, but there was no evidence of a close or special relationship between nerves with VIP or SP and these cells. The absence of close relationships between nerves and either interstitial cells or smooth muscle cells leaves unanswered questions about the structural basis for previous observations of discrete excitatory responses or pyloric sphincter to single stimuli or nerves up to one per second. In conclusion, the structural observations suggest that this muscle has special neural and myogenic control systems and that interstitial cells may function to control myogenic activity of this muscle but not to mediate neural signals.Key words: vasoactive intestinal polypeptide, interstitial cells of Cajal, neuropeptides, gap junctions, substance P.