Background: The digestive hydrolysis of dietary proteins leads to the release of peptides in the intestinal tract, where they may exert a variety of functions, but their characterization and quantification are difficult. Objectives: We aimed to characterize and determine kinetics of the formation of peptides present in the jejunum of humans who in-gested casein or whey proteins by using mass spectrometry and to look for and quantify bioactive peptides. Design: Subjects were equipped with a double-lumen nasogastric tube that migrated to the proximal jejunum. A sample collection was performed for 6 h after the ingestion of 30 g 15 N-labeled casein (n = 7) or whey proteins (WPs; n = 6). Nitrogen flow rates were measured, and peptides were identified by using mass spectrometry. Results: After casein ingestion, medium-size peptides (750–1050 kDa) were released during 6 h, whereas larger peptides (1050–1800 kDa) were released from WPs in the first 3 h. A total of 356 and 146 peptides were detected and sequenced in the jejunum after casein and WP ingestion, respectively. b-casein was the most important precursor of peptides, including bioactive peptides with various activities. The amounts of b-casomorphins (b-casein 57–, 58–, 59–, and 60–66) and b-casein 108–113 released on the postprandial window were sufficient to elicit the biological action of these pep-tides (ie, opioid and antihypertensive, respectively). Conclusions: Clear evidence is shown of the presence of bioactive peptides in the jejunum of healthy humans fed casein. Our findings raise the question about the physiologic conditions under which these peptides can express their bioactivity in humans. This trial was registered at clinicaltrials.gov as NCT00862329. Am J Clin Nutr