1. Samples of portal venous and femoral arterial plasma and mesenteric lymph were collected from unanaesthetized rats before and during the intra-duodenal infusion of high concentrations (100 mmol/l) of glycine or glycine oligopeptides (up to and including tetraglycine). 2. There was a marked increase in the concentration of free glycine, and a small amount of diglycine was detected in all the fluids when the peptides were infused but not during the infusion of free glycine. No other glycine oligopeptides were detected. 3. Mesenteric lymph contained a similar concentration of glycine and diglycine to that found in portal blood, suggesting that the mesenteric lymph is a dialysate of portal plasma. 4. The results are interpreted as supporting a dual localization of peptidases within the enterocyte; glycine oligopeptides are hydrolysed to diglycine and free glycine at the brush border and the diglycine is hydrolysed within the cytoplasm.