Transport of [14C]pantothenic acid was studied using brush-border membrane vesicles prepared from rat kidney. In the presence of a Na+ gradient an accumulation of pantothenic acid 3-fold above equilibrium was observed. The Km and Vmax found were 7.30 microM and 23.8 pmol/mg protein per min, respectively. Isolated perfused rat kidneys were employed to study excretion of pantothenic acid at various concentrations in the perfusate. At physiological plasma concentrations, the filtered pantothenic acid was largely reabsorbed by the active process observed in the vesicles. At higher concentrations, pantothenic acid was found to undergo tubular secretion. Penicillin inhibited this secretory process indicating that both compounds share a secretory mechanism. Live animal studies indicated that the only compound excreted after injection of [14C]pantothenic acid was free pantothenic acid. After 1 week only 38% of the administered dose was excreted in the urine, indicating that effective conservation was taking place in the whole animal.