1. Glycyl-L-proline and L-prolyl-glycine are two dipeptides that are poorly hydrolysed by brush border peptide hydrolases in vitro. The present study was undertaken to investigate in vivo intestinal absorption of the two dipeptides in male Wistar rats. A steady-state perfusion technique has been used and jejunal and ileal absorption studied by comparing rates of absorption of constituent amino acids from dipeptide and equivalent equimolar amino acid solutions. 2. Glycine and proline were absorbed at the same or faster rates during perfusion of 10 mM glycyl-L-proline than during perfusion of 10 mM-L-prolyl-glycine than during perfusion of its equivalent equimolar amino acid solution. Conversely, the two constituent amino acids were absorbed at slower rates during perfusion of its equivalent amino acid solution. As expected very low concentrations of free amino acids were detected during the jejunal dipeptide perfusions but higher concentrations were detected during the ileal perfusions. 3. The addition of 40 mM L-prolyl-glycine to the 10 mM glycyl-L-proline solution did not affect the rates of disappearance of glycyl-L-proline. The addition of 40 mM glycyl-L-proline to 10 mM L-prolyl-glycine however had a significant inhibitory effect on the disappearance of L-prolyl-glycine. It is concluded that the two dipeptides may be absorbed by different peptide transport systems.