The pharmacology of morphine and opioid peptides was studied in the guinea-pig ileum by examining their inhibitory effects on propulsive peristaltic activity and on the cooling-induced longitudinal contraction. In these experiments, dose-response curves were recorded. The rank order of potency in inhibiting peristalsis was found to be: dermorphin greater than FK 33-824 greater than dynorphin-(1-17) greater than dynorphin-(1-13) greater than delta-receptor-peptide greater than morphine greater than [Leu] enkephalin, whereas the rank order in inhibiting cooling-induced contractions was found to be: dynorphin-(1-13) congruent to FK 33-824 congruent to dermorphin greater than delta-receptor peptide greater than morphine. Naloxone antagonized the maximally effective dose of each of the opioid agents. In view of the differences between the abilities of these opioids to inhibit propulsive peristaltic activity, these models seem to be valuable for the examination of inhibitory opioid effects in the gut.