The main objectives of this work have been to study (1) whether or not opioid effects on gastrointestinal motility in chicken are mediated through central or peripheral pathways; (2) the receptors involved; (3) the correlation of the motor response to the distribution of Met-enkephalin in the gastrointestinal tract and (4) to evaluate the physiological role of endogenous opioids in spontaneous MMCs. Intravenous infusion of Met-enkephalin and morphine (5 x 10(-7) mol/kg) induced gastric inhibition and a migrating intestinal hyperactivity. Induced intestinal activity was faster in vagotomized chickens. In the stomach there was a correlation between the duration of the inhibitory response and the affinity of the agonists for mu-receptors ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO) > morphine > Met-enkephalin > Tyr-D-Pen-Gly-Phe-D-Pen (DPDPE)). DPDPE induced duodenal hyperactivity which was not propagated. Immunohistochemistry showed that Met-ENK like material is mainly located at the myenteric plexus and the outer circular muscle in stomach. In the intestine, it was found in the myenteric and the deep muscular plexuses. When endogenous MMCs were studied, i.v. infusion of naloxone lengthened significantly their duration. In conclusion, the fact that Met-enkephalin and their analogs induced a migrating activity in the intestine and the lengthening of MMC by naloxone, suggest a physiological role for opioids on induction of MMC in birds, acting at peripheral level.