The effect of glomerulopressin was investigated in several isolated veins and arteries of dog and also in veins of rabbit, rat, and hamster. Glomerulopressin contracted some of the dog veins: jugular, extrahepatic porta, femoral, cava, iliac, splenic, and had no effect in pulmonary, mesenteric, and renal vein. It was also active in some dog arteries: Iliac, femoral, renal, and had no effect in aorta, hepatic, splenic and pulmonary artery. In the rat it increased the frequency of the spontaneous rhythmic contraction of the extrahepatic porta. No effect was observed in any of the rabbit veins assayed: cava, jugular, iliac and extrahepatic porta, nor in the extrahepatic portal vein of the hamster. In another group of experiments, inhibitors of phospholipase A2, chlorpromazine and mepacrine were added to the bath. These inhibitors blocked the effect of glomerulopressin in the three dog vessels that were assayed: jugular, extrahepatic porta and iliac artery. These results show that glomerulopressin induces a contraction in several, but not all, veins and arteries, and that its effect is mediated by the liberation of arachidonic acid.