The present study was intended to examine the effect of sodium vanadate on contractility of vascular smooth muscle. Aortic ring preparations were made from the rabbit thoracic aorta and endothelial cells were removed from the ring. The contractility of the aortic ring was measured under various conditions. The results were summarized as follows; 1) Sodium vanadate induced contraction of vascular smooth muscle in a dose-dependent fashion. 2) The contractile effects were not blocked by treatments with adrenergic blocking agent(phentolamine) and indomethacin, indicating the direct action of the drug on vascular smooth muscle. 3) In the presence of ouabain, Na⁺-K⁺-ATPase inhibitor, sodium vanadate still increased the contractility of vascular smooth muscle. 4) Treatment with 4.4'-diisothiocyanostilbene-2.2'-disulfonic acid(DIDS) blocked completely the contractile effects of sodium vanadate. 5) In the presence of verapamil, lanthanum and ryanodine, the contractility of the vascular smooth muscle by sodium vanadate was decreased. From the above results. it was suggested that sodium vanadate acts directly on vascular smooth muscle and causes contraction. It was probably due to inhibition of Ca⁺⁺-ATPase in plasma membrane as well as increasing the release of Ca⁺⁺ from sarcoplasmic reticulum and Ca⁺⁺ influx across the plasma membrane, but not inhibition of Na⁺-K⁺-ATPase.