Transport of intact peptides, followed by intracellular hydrolysis in the intestinal mucosal cells, plays an important role in the absorption of protein digestion products in the mammalian small intestine. Even though earlier studies on peptide absorption in intact-tissue preparations have indicated that peptides are transported by an active Na+-dependent mechanism, recent studies with purified brush-border membrane vesicles have unequivocally demonstrated that Na+ does not play a direct role in the translocation of peptides across the membrane. Like most amino acids, peptides are also transported as zwitterions. However, peptide transport causes depolarization of the brushborder membrane in intact mucosal cells as well as in purified membrane vesicles, and the depolarization is the result of a net transfer of positive charge across the membrane during peptide transport. This electrogenic nature of peptide transport is observed even in the absence of Na+. Peptide transport is enhanced by an interior-negative membrane potential and inhibited by an interior-positive membrane potential. An inward proton gradient stimulates peptide transport, and this stimulation is reduced when the proton gradient is subjected to rapid dissipation by the presence of a proton ionophore. These observations strongly suggest that peptides are cotransported with protons in the intestine. There is substantial evidence for the existence of an inward proton gradient in the mammalian small intestine, and therefore it is very likely that this proton gradient is the in vivo energy source for the uphill transport of peptides. The Na+-H+ exchanger in the brush-border membrane, in conjunction with Na+-K+-ATPase at the basolateral membrane, is probably responsible for the generation and maintenance of the proton gradient and may thus be involved indirectly in the intestinal absorption of peptides.