In acute experiments in anesthetized cats (n = 75), with ligated adrenals the influence on the pyloric sphincter of afferent and efferent electrical stimulation of the cervical vagi was studied with a flow recording technique. Gastric motility was recorded simultaneously with a volume recording technique. Efferent stimulation using high threshold parameters (8 V; 5 ms; 8 Hz) resulted in a prompt gastric contraction and a delayed pyloric contraction (23 +/- 17 s). In one third of the cats biphasic pyloric motor response, consisting of a short period of increased flow preceding a longlasting cessation of the flow, was observed. When the transpyloric flow was reduced by splanchnic nerve stimulation or noradrenalin infusion, efferent vagal nerve stimulation induced an increased flow supporting the assumption of relaxatory fibres to the pylorus within the vagus as well. Unilateral afferent stimulation resulted in a decreased flow within 5 +/- 3 s and a prompt gastric relaxation, which were both abolished at transection of the intact contralateral vagus indicating vago-vagal reflexes. After atropine (0.2 mg/kg i.v. b.w.) efferent stimulation resulted in a gastric relaxation, while the biphasic pyloric motor response was even more pronounced with a long (60 s) latency of the contractile phase. Addition of guanethidine (2 mg/kg i.v. b.w.) did not affect these responses. After hexamethonium (25 mg i.v. + 50 mg +/- 10 mg i.a./h) the stimulation procedure resulted in a gastric relaxation, while the pyloric contraction was blocked. The relaxatory phase required the addition of atropine for blockade, indicating separate transmission mechanisms for the two components of the pyloric motor response at such stimulation. Hexamethonium effectively antagonized the pyloric contraction at afferent stimulation. Immunohistochemical studies revealed a rich enkephalinergic innervation of the pylorus and the presence of enkephalin-like material within vagal axons. VIP-like material was demonstrated within the same tissues with a similar distribution. The hypothesis of a transmission via non-classical receptors was corroborated in experiments, where the pyloric contraction at vagal efferent stimulation was blocked dose-dependently by the opiate receptor antagonist, naloxone i.a. Furthermore, micromolar doses i.a. of enkephalins mimicked the vagal motor effects, i.e. pyloric contraction and gastric relaxation, which were reversed by equimolar doses of naloxone in favour of a pyloric enkephalinergic contraction. Intraarterial injection of nanomolar doses of this VIP resulted in both a gastric and pyloric relaxation. Specimens of the human vagus from the thoracic and abdominal levels were investigated with immunohistochemistry. Substance P and enkephalin containing fibres and a low number of VIP fibres were seen at all levels indicating the presence of an axonal transport of these peptides in the human vagus.