1. The intestinal absorption of carnosine, glycylglycine, glycyl-d-phenylalanine, glycyl-l-phenylalanine, glycyl-l-proline and l-prolylglycine were investigated after intraluminal injection of dipeptide into anaesthetized rats.
2. With all six dipeptides, the intact substance was detected by ion-exchange chromatography in blood samples taken from the superior mesenteric vein.
3. The rate of hydrolysis of the dipeptides in tissue homogenates was measured in vitro.
4. The relative rates of hydrolysis varied by a factor of 300; there was an apparent inverse relationship between rate of hydrolysis and detection of intact peptide.
5. Peptide absorption was accompanied by increases in venous concentrations of the component amino acids, which appeared in proportions appropriate to the view that peptide absorption preceded hydrolysis.
6. It is suggested that slowly hydrolysed dipeptides may pass intact through the intestine wall under physiological conditions.