The presence of motilin in human milk and the influence of human milk on the degradation of [125I][Nle13] porcine motilin by gastric and duodenal fluids were investigated. Milk and plasma samples were collected from 14 mothers, and motilin was measured by radioimmunoassay. Plasma levels were 416 +/- 37 pg/mL. In 8 defatted samples the motilin level was 105 +/- 14 pg/mL, in the six others levels were above 1000 pg/mL but dilution curves were non-linear. After solid-phase extraction milk levels were 108 +/- 21 pg/mL in 13 samples, in 1 sample the dilution curve was still non-linear. The stability of motilin after ingestion was studied in vitro by incubating [121I][Nle13] porcine motilin with gastric and intestinal juices obtained from newborns (10 times diluted). Incubations were performed at 37 degrees C at pH 1.8, 3.2 and 5.8 for the gastric fluid and at pH 7.4 for the duodenal fluid. After different times of intervals (5, 10, 20 and 30 minutes) intact motilin was precipitated with trichloroacetic acid and the radioactivity of the supernatant was determined. Motilin was rapidly degraded by gastric juice. The breakdown was greatest at pH 3.2 (74% after 30 minutes) and lowest at pH 5.8 (29%), the pH after milk feeding in neonates. Degradation by intestinal juice at pH 7.4 was also very rapid (77% after 30 minutes). Human milk and BSA inhibited partially the gastric digestion at pH 3.2 (17 and 29%, respectively). Digestion by intestinal juice was not affected by human milk and BSA. These results suggest that digestion of motilin in the stomach may be sufficiently retarded by human milk in the newborn to exert a biological role.