The relation between arsenate and phosphate transport in the dog kidney was studied by measuring the renal clearance of arsenate labeled with its radioactive isotope As74. The experiments were performed during osmotic diuresis induced by mannitol. The results demonstrate certain similarities in the transport of these ions. Arsenate undergoes a net tubular reabsorption which is inhibited as the plasma phosphate concentration is raised. The inverse relationship between arsenate transport and the plasma As:P ratio suggests a competitive mechanism for the interaction between the two ions Like phosphate, arsenate transport is inhibited by glucose and this effect is reversed by phlorizin. An important difference between arsenate and phosphate transport is the sensitivity of arsenate transport to urine flow. In vivo reduction of arsenate to arsenite and a net tubular secretion of arsenite has been observed. The results are discussed in terms of the known ability of arsenate to substitute for phosphate in biochemical reactions.