Kidney function changes after single-dose administration of borocaptate sodium were studied in rats and in patients with brain tumors. Changes of glomerular filtration rate (GFR) measured as 14C-inulin clearance and urine flow rate (UFR) after a slow intravenous injection of BSH (25 and 50 mg/kg b.w., respectively) were investigated in rats under pentobarbital anesthesia. The effect of BSH has been compared with that of its disulfide (BSSB) which is spontaneously generated by oxidation of BSH during storage. It was found that BSH decreases GFR in relation to dose and, in the same way, causes a temporary increase of UFR. On the other hand, BSSB (50 mg/kg) induced a large reversible decrease of GFR as well as a decrease of urine excretion. Measurements of GFR (inulin clearance), renal plasma flow (PAH clearance) and urine excretion were taken in a group of patients with brain tumors in which boron disposition after an infusion of BSH (25 mg/kg b.w. over 1 h) had been studied. An increase in urine production was the dominant effect (up to 200% of the initial value), with the alterations of GFR and RPF being of minor significance except in one patient with a GFR reduction up to almost 50% the original value. Kidney function changes after BSH or BSSB administration are supposedly related to the high retention of BSH in kidney.