A new analog of prostacyclin, 6,9-Thiaprostacyclin was infused intravenously in pentobarbital anesthetized cats in order to determine its hemodynamic and anti-platelet aggregating properties. At an infusion rate of 0.01 μmoles/kg/min, PGI2-S moderately decreased arterial blood pressure without altering heart rate of superior mesenteric artery flow or platelet aggregation responses to ADP. However, at 0.05 μmoles/kg/min, PGI2-S significantly reduced arterial blood pressure and significantly increased heart rate, and superior mesenteric artery flow. Moreover, at 0.05 μmoles/kg/min, PGI2-S inhibited ADP platelet aggregation by 80%. PGI2-S may be a useful agent in circulatory shock.