Influxes of glycyl-L-proline (a dipeptide which is not hydrolysed in the membrane and is transported intact across the brush border) and of glycyl-L phenylalanine (a dipeptide which has affinity for the peptide transport system and is hydrolysed at the brush border membrane) have been studied in the small intestine of fetal, newborn and suckling rabbits. For glycyl-L-phenylalanine, transport as the intact dipeptide and 'membrane hydrolysis + amino acid transport' have been measured separately by using glycyl-L-proline and L-leucine as selective inhibitors of each pathway. For comparison, uptake of free glycine and of free phenylalanine has also been studied. The intestine of newborn rabbits is shown to have a translocation process for intact dipeptides which is saturable with a low Kt and stimulated by sodium ions, and which is not shared by free amino acids. This process resembles that described in adult animals, except that the maximal velocity is much higher in newborns. The developmental pattern of this uptake process for dipeptides differs markedly from that of free glycine, thus providing a new type of evidence for the distinction between amino acid and dipeptide transport processes. The developmental pattern of the free phenylalanine uptake process also differs from the development of the 'superficial hydrolysis + amino acid transport' component of glycl-L-phenylalanine uptake. These data suggest that the advantage of mucosal uptake of peptides, compared to the uptake of free amino acids, is much greater in the early stages of postnatal life than in the adult.