1. In chloralose-urethane-anasthetized dogs a manometric assembly was inserted via a gastrostomy to monitor pyloric pressure with a sleeve sensor. Antral and duodenal contractions were monitored with both manometric side holes and serosal strain gauges. 2. Subserosal silver wire electrodes were placed in the antrum 5 cm orad and the duodenum 3 cm aborad to the pylorus to facilitate field stimulation of intrammural nerves. 3. The pylorus exerted spontaneous tone (10.8 .+-. 4.8 mm Hg) with phasic contractions occurring at a rate varying from 1-5 min-1 and, at times, with a superimposed higher frequency up to 15 min-1. Atropine (30 .mu.g kg-1 I.V. and 10 .mu.g I.A.) reduced and tetrodotoxin (50-100 .mu.g I.A.) enhanced the phasic activity significantly. 4. Bilateral cervical vagal had no consistent influence on pyloric motility. 5. Stimulation of the distal ends of the cervical vagal nerves at low frequences (0.2-0.5 Hz, 1-3 ms, 20 V) induced phasic pyloric contractions, which were abolished by atropine or hexamethonium (10 mg kg-1 I.V. and 1 mg I.A.). Higher frequencies (>0.7 Hz) of stimulation inhibited both phasic and tonic contractions and this inhibition was unaffected by atropine, hexamethonium, phentolamine (1.5 mg kg-1 I.V. an 100 .mu.g I.A.) or propranolol (1 mg kg-1 I.V. and 100 .mu.g I.A.). All neural responses were blocked by tetrodotoxin (50-100 .mu.g I.A.). 6. Duodenal field stimulation (0.2-5 Hz, 0.5 ms, 40 V) induced strong phasic and tonic contractions in the pylorus. This excitation was blocked by atropine, hexamethonium, tetroditoxin (50-100 .mu.g I.A.) or duodenal transection orad to the stimulating electrodes. 7. Antral field stimulation (0.5-1 Hz, 0.5 ms, 40 V) completely abolished phasic activity in the pylorus and reduced toic activity, regardless of whether the contractile activity was spontaneous or induced by neural stimulation. This inhibitory action was unaffected by atropine, hexamethonium or propranolol but was blocked by tetrodotoxin and antral transection aborad to the the stimulating electrodes. Phenotolamine attenuaed the inhibitory effect of antral field stimulation on pyloric motility. 8. It is concluded that the distal canine pylorus exhibits myogenic tone and phasic activity which is modulated by extrinsic and intrinsic nerve pathways. Vagal nerves contain fibers, activated by different stimulus parameter which can either excite or inhibit pyloric activity. Activation of antral nerves inhibits pyloric activity, with both non-adrenergic, non-cholineric and phentolamine-sensitive pathways contributing to this inhibitory response. Activations of duodenal intrinsic nerves activates a chain of orally projecting cholinergic nerves which enhances pryloric activity. The pyloric muscle should be considered as sphincter because of its myogenic activity and special neural control.
