Assimilation of systemic oligopeptides (di- and tripeptides) is largely a function of kidneys. The most specific and unique mechanism utilized for the performance of this renal function is transport, followed by intracellular hydrolysis and then release of constituent amino acids to the systemic circulation. Among tissues examined (liver, kidney, intestine, and muscle), kidney is the only tissue capable of accumulating dipeptides in concentrations that are greater than their plasma concentrations. Kidney also is the tissue with the highest cytoplasmic dipeptidase activity. Intracellular accumulation is mediated by two transporters (Pept-1 and Pept-2), both of which have been recently cloned. These transporters use dipeptides and tripeptides as substrates and rely on protons and membrane potential for their driving force. Pept-1 is a low-affinity, high-capacity transporter, and Pept-2 is a high-affinity, low-capacity transporter. The nutritional and metabolic regulation of renal assimilation of oligopeptides is suggested by the selective decrease in dipeptide balance across the kidneys of starved human subjects and by the insulin stimulation of dipeptide transport by a renal cell line. Peptiduria has been observed in a variety of diseases, but the mechanism, except in genetic diseases affecting hydrolysis of oligopeptides, is not known. Finally, the capacity for active transport of oligopeptides and peptidomimetic drugs enables kidneys to play major roles in nutritional and pharmacological therapies.