Ro 15-2041 inhibits platelet aggregation induced by all common platelet agonists in platelet-rich plasma obtained from various species including man with IC50 of 1-3 μM. It potentiates platelet inhibition by prostacyclin, the prostacyclin-induced increase of intraplatelet cyclic (c) AMP levels and inhibits the collagen-induced release of serotonin and beta-thromboglobulin. Ro 15-2041 reduces the increase and accelerated the normalization of cytosolic free Ca2+ in thrombin-stimulated human platelets. Ro 15-2041 is a potent (IC50=70 nM) and selective inhibitor of platelet cAMP-phosphodiesterase activity.