Carbocyclic thromboxane A2 [2 beta (Z),3 alpha- (1E,3R*)-3-(3-hydroxy(1-octenyl)-bicyclo[3.1.1]hept-2-yl-5-heptenoic acid], a stable analog of thromboxane A2, has been tested for its physiologic properties. Carbocyclic thromboxane A2 is a potent coronary vasoconstrictor, stimulating cornonary vascular smooth muscle at concentrations as low as 29 pM. At 1-5 micro M it is also an inhibitor of arachidonic-acid- and endoperoxide-induced aggregation of platelets. At 200 nM it stimulated the release of lysosomal hydrolases from large granule fractions of liver homogenate. It inhibited thromboxane synthesis in platelets, although it did not inhibit synthesis of prostacyclin in ram seminal vesicles. Thus, carbocyclic thromboxane A2, a molecule closely related to thromboxane A2, separates coronary vasoconstrictor from platelet-aggregating activity. The constrictor activity predominates in vivo; carbocyclic thromboxane A2 induces coronary vasoconstriction leading to myocardial ischemia and sudden death in rabbits in the absence of pulmonary or coronary thrombosis.