The first months of life correspond to a key period in human life where dramatic physiological changes (establishment of microbiota, development of the immune system, etc.) occur. In order to better control these changes it is necessary to understand the behaviour of food in the gastrointestinal tract of the newborn. Infant formula is the only food for the newborn when breast-feeding is impossible. The kinetics of digestion of milk proteins and the nature of the peptides liberated in the small intestine throughout infant formula digestion have never been extensively investigated so far and were therefore studied using the piglet as a model of the newborn child. Piglets were fed infant formula by an automatic delivery system during 28 d, and slaughtered 30, 90 and 210 min after the last meal. Contents of stomach, proximal and median jejunum and ileum were collected and characterised. The extent of β-lactoglobulin (β-lg), α-lactalbumin (α-la) and casein proteolysis was monitored by inhibition ELISA, SDS-PAGE, immunoblotting and MS. At 30 min after the last meal, caseins were shown to be extensively hydrolysed in the stomach. Nevertheless, peptides originating mainly from β-caseins (from 509 to 2510 Da) were identified in the jejunum and ileum of the piglets. β-Lg partially resisted gastric digestion but completely disappeared in the stomach after 210 min. α-La had a similar behaviour to that of β-lg. Two large peptides (4276 and 2674 Da) generated from β-lg were present in the ileum after 30 and 210 min and only one (2674 Da) after 90 min.