Both the vagus nerve and motilin have been implicated in the initiation of phase III of the fasting migrating motor complex. To investigate the contribution of each, the effects of intravenous porcine motilin, vagosympathetic nerve blockade, and atropine were assessed. Intralumenal pressures of the lower esophageal sphincter, stomach, and upper small intestine, and plasma motilin levels were monitored. Porcine and canine motilin in plasma were distinguished by radioimmunoassay using two different antibodies. Injection of porcine motilin (75-370 pmol), initiated dose-dependent phase III-like motor activity in the lower esophageal sphincter, stomach, and small bowel if the vagi were intact; if the vagi were blocked, activity was initiated in the small bowel only. Moreover , a significant (p less than 0.001), dose-dependent peak in canine plasma motilin was observed after the onset of the induced motor activity when the vagi were intact or blocked, with plasma motilin peaks comparable to those occurring spontaneously. In both intact and vagally blocked dogs, atropine abolished both the spontaneous motor activity and associated rise in motilin level, and also abolished porcine motilin-induced activity. However, a diminished, but significant (p less than 0.01) peak in porcine motilin-induced canine motilin persisted in the presence of atropine. These results in dogs indicate that while spontaneous phase II motility in the upper gastrointestinal tract and phase III activity in the lower esophageal sphincter and stomach are dependent on vagally mediated excitatory pathways, spontaneous and induced phase III motor activity in the small intestine are dependent on nonvagal cholinergic innervation. Canine motilin release induced by porcine motilin is mediated primarily by a nonvagal cholinergic (muscarinic) pathway, with minor contributions from vagal noncholinergic, and nonvagal noncholinergic mechanisms. Because spontaneous or induced motilin release peaks well after the onset of phase III motor activity, it is unlikely that motilin is the primary factor responsible for initiation of the migrating motor complex in the dog. Motilin may, however, modulate motility produced by preexisting neural excitation.