Exhausted rat aorta slices were incubated for 20 minutes at room temperature with plasma from 6 uremic patients undergoing maintenance hemodialysis, from 23 patients with hepatic disorders and with plasma from healthy volunteers. We observed that plasma from uremic as well as from patients with liver dysfunction stimulated prostacyclin release more than the simultaneously studied normal plasmas. Enhanced prostacyclin release may play a role in the pathogenesis of the hemorrhagic diathesis of patients with renal or hepatic failure.