Evidence for role of prostacyclin as a systemic hormone in portal hypertension. The possibility that prostacyclin could be a systemic hormone and could mediate the splanchnic hyperemia of chronic portal hypertension was evaluated in rabbits in a normotensive state and in rabbits with chronic partial ligation of the portal vein. In rabbits with portal hypertension (PHT), 6-keto-prostaglandin F1 alpha (PGF1 alpha, a prostacyclin degradation product) was elevated twofold in all vascular beds (systemic arterial, systemic venous, and portal venous) when compared with levels in control animals. In PHT rabbits, exogenous prostacyclin infusion after cyclooxygenase blockade through the systemic arterial, systemic venous, or portal venous route resulted in an equal elevation of 6-keto-PGF1 alpha in the reciprocal vascular beds and restored the original precyclooxygenase blockade hemodynamics. These hemodynamic changes were of equal magnitude irrespective of site of infusion in PHT. In controls there was no significant change in 6-keto-PGF1 alpha or hemodynamics with intraportal infusion. We conclude that prostacyclin achieves systemic levels by escaping hepatic degradation resulting from portosystemic shunting in the animal with chronic portal hypertension. 