The cardiovascular actions of the newly discovered bicyclic prostaglandin, prostacyclin, or PGI2, were compared with those of PGE1 in the anesthetized cat. PGI2 decreased systemic arterial pressure, increased cardiac output and decreased systemic vascular resistance. The decreases in systemic vascular resistance were similar when PGI2 was injected into the left or right atrium, suggesting that prostacyclin is not inactivated in the feline pulmonary vascular bed. PGE1 also decreased systemic arterial pressure and increased cardiac output; however, left atrial administration of this substance produced greater reductions in systemic vascular resistance than right atrial injections, suggesting that PGE1 is inactivated in the feline lung. PGI2 also caused dose-related decreases in perfusion pressure in the renal, hindquarters and mesenteric vascular beds and had the greatest vasodilator activity in the mesenteric vascular bed. PGE1 decreased perfusion pressure in the 3 regional beds, and the overall dilator effects of PGI2 and PGE1 in the peripheral circulation were quite smiliar. The present data show that PGI2 is a potent peripheral vasodilator in the cat and since this substance is not inactivated in the lung, it could serve as a circulating hormone in this species.