The properties of glycylglycine and glycine transport across the mucosal border of the epithelium were studied in isolated guinea pig ileum. The initial influxes of both substances obeyed simple Michaelis-Menten kinetics, but the values of the kinetic parameters were greatly different between two substances; Jmax and Kt for glycine were 290 nmol.cm-2.min-1 and 27 mM, and those for glycylglycine were 57.4 nmol.cm-2.min-1 and 3.4 mM, respectively. Total replacement of Na+ in the incubation medium with mannitol completely suppressed the cellular uptake of glycine, while it reduced glycylglycine uptake only slightly. The omission of Na+ caused a slight increase in Kt for glycylglycine (from 3.4 mM at 100 mM Na+ t0 5.2 mM at 0 mM Na+) without changing the value of Jmax. Glycylglycine taken up by the epithelial cells had been completely hydrolysed into glycine within 2-min incubation period. The results of the present study seem to indicate that there are entirely distinct transport systems for glycine and its dimer and that the membrane digestion and subsequent transport as the monomer may not be involved in the absorption of glycylglycine.