Peptide products insoluble in 12% TCA and obtained in the proximal duodenum of calves at different times after ingestion of casein solutions were characterized by polyacrylamide gel electrophoresis, electrofocusing and SDS pore gradient gel electrophoresis. With a 3% whole-casein solution in water the disappearance of electrophoretic bands corresponding to alpha s1 and beta caseins was observed after about 1 h 30. After 3 h and up to 7 h very acidic peptides appeared. With a 3% whole-casein solution in simulated milk ultrafiltrate, comparable patterns were obtained. Nevertheless, the acidic peptides appeared sooner, i.e. they were already detected in the first sample collected after meal ingestion. With the 2 diets, the importance of gastric proteolysis was demonstrated by the appearance of a great number of peptides of various sizes, charges and pHi. On the other hand, an effect of salts on casein proteolysis was detected.