Vanadium is widely distributed in animal tissues and it is supposed to be a regulator of Na-K-ATPase activity. The effect of sodium orthovanadate on Na-K-ATPase activity in rabbit kidney was measured in vitro and compared with that of ouabain. The influence of sodium orthovanadate on the renal function of rabbits was also investigated. 1) Na-K-ATPase activity was decreased by sodium orthovandate at the concentrations of 10^-7, 10^-6, 10^-5 and 10^-4 M to 73.89, 36.49, 6.50 and 4.99% of the control activity respectively. 2) Na-K-ATPase activity was decreased by ouabain at the concentrations of 10^-4, 10^-3 and 10^-2 M to 69.52, 22.84 and 3.88% of the control activity respectively. 3) Urine volume, urinary excretion of Na⁺, K⁺ and Cl^-, clearances of inulin and p-amino-hoppuric acid were decreased until after 60 minutes following the administration of sodium orthovanadate 0.5 mg/kg intravenously Na⁺ reasorption rate was not changed and mean arterial pressure was significantly elevated during 60 minutes after the administration of sodium orthovanadate.