Summary: The influxes from the mucosal solution to the enterocytes were investigated for glycylglycine, glycyl-L-leucine, and their constituent amino acids in isolated intestinal segments from suckling (3 to 4 days), weanling (10 to 14 days), and adult guinea pigs. Glycine and L-leucine influxes from the mucosal solution containing free glycine or L-leucine were not significantly different between sucklings and adults in either the jejunum or the ileum. On the other hand, glycylglycine and glycyl-L-leucine influxes were significantly greater in the following sequence: sucklings > weanlings > adults. Such age-dependent difference in dipeptide influx was more prominent in the jejunum than in the ileum. Kinetically, such developmental change in dipeptide influx was found to be related to the change in the value of Jmax (the maximum influx). The data indicated that the intact-peptide transfer mechanism has more important significance in protein hydrolysate absorption in immature stages of animals than in adults. Speculation: Age-dependent change in dipeptide influxes into the enterocytes may be related to the change in the density of carriers for intact peptides at the brush border membranes. It is also speculated that the greater importance of the intact-peptide transport may have some relation to immature intracaval (intermediate) digestion or less effective functional linkage between membrane surface hydrolysis and amino acid transport in suckling and weanling stages.