It has been hypothesized that humans with familial renal hypouricemia may have a generalized defect of urate transport across cell membranes due to the genetic deletion of a specific carrier, a defect similar to that reported in the Dalmatian dog. In this study the transport of urate labelled with carbon 14 by the erythrocytes of four patients with familial renal hypouricemia was identical to that of five healthy controls. The addition of hypoxanthine to the incubation medium inhibited the transport to a similar extent in the two groups of patients, demonstrating the presence of a carrier specific for urate. This carrier was also found to be present in the erythrocytes of Dalmatian and mongrel dogs. Thus, the renal anomaly causing the hypouricemia in both species is not related to a generalized deletion of a urate-transporting protein on cell membranes.