Healthy young volunteers ingested one liter of cows' milk; then the contents of the small intestine were aspirated through an intestinal tube at various times and assayed for the presence of bovine beta-casomorphin immunoreactive materials. Considerable amounts of beta-casomorphin-7, but no beta-casomorphin-5 and only small amounts of beta-casomorphin-4 or -6 immunoreactive materials were found. Chromatographical characterization showed that most of the beta-casomorphin-7 immunoreactive material was not identical with beta-casomorphin-7, whereas the major part of the beta-casomorphin-4 or -6 immunoreactive materials might be identical with their corresponding beta-casomorphins. Analogous results were obtained for in vitro digestion of bovine milk which had been designed as a rough imitation of the gastrointestinal digestion process. A regulatory influence of beta-casomorphins as "food hormones" on intestinal functions is suggested.