U-51605 is a stable analog of the endoperoxide prostaglandin H2 (PGH2). It is an inhibitor of both prostacyclin (PGI) and thromboxane (TX) synthases with more selectivity towards PGI synthase. U-51605 is also a partial agonist at TP receptors. In human foreskin fibroblasts, U-51605 inhibits PGI synthase at a concentration of 2.8 µM, whereas, human platelet TX synthase is inhibited at a concentration of 5.6 µM. At concentrations up to 1 µM, U-51605 reduced the release of prostacyclin in SHR aorta elicited by the calcium ionophore A-23187 with no effect on TXA2 production and yet significantly increased PGE2 and PGF2α release.