Vasodilation of small blood vessels is controlled in part by the endothelium-derived relaxing factor (EDRF), which also inhibits platelet adhesion. Methylene blue (MB), which is occasionally applied directly to blood vessels during microsurgery to provide orientation and prevent torsion, is an irreversible inhibitor of the effects of endothelium-derived relaxing factor and may thereby augment both vasospasm and platelet responses. We have investigated the effects of the extravascular adventitial application of methylene blue on platelet deposition to human placental arteries (HPA) in the presence and absence of surgically induced vasospasm. A trend toward increased platelet deposition to human placental arteries was seen in each group but did not reach significance. The degree of platelet deposition to control human placental arteries suggests that the effects of methylene blue on platelet deposition may be dwarfed by the effects of surgical trauma and ischemia.