Carbacyclin is a chemically stable analogue of prostacyclin. As an inhibitor of platelet aggregation induced by ADP or collagen in vitro, carbacyclin is 0.03 times as active as prostacyclin in human, dog or rabbit plasma. Carbacyclin, like prostacyclin, reduces systemic arterial blood pressure (BP) in dogs, rabbits and rats and is not inactivated during passage through the pulmonary circulation. Further actions were investigated using a new ex vivo technique which allows rapid preparation of platelet-rich plasma and determination of platelet aggregation. In the dog, intravenous infusion of carbacyclin or prostacyclin inhibits platelet aggregation ex vivo with minimal effects on BP or heart rate. In the anaesthetised or conscious rabbit, carbacyclin and prostacyclin produces similar cardiovascular changes in doses producing an equivalent degree of platelet inhibition. In both rabbit and dog, carbacyclin is 0.1 times as active as prostacyclin in inhibiting ex vivo platelet aggregation. Platelet inhibition is maintained throughout the period of infusion of either compound (up to 3 h) yet is no longer apparent 10 min after terminating the infusion. Carbacyclin is thus a chemically-stable but metabolically-unstable analogue with a biological profile closely similar to prostacyclin.