We evaluated the antioxidant property of melatonin as it relates to the vasorelaxant effect of peroxynitrite (ONOO ), a reaction product of superoxide anion radical (O2(-*)) and nitric oxide (NO), on the human umbilical artery. Helical sections of umbilical arteries were obtained from human placentas at elective cesarean delivery between weeks 37 and 39 of gestation. Changes in maximal tension induced by potassium chloride were measured in arterial sections with intact endothelium. Sections were treated with 3-morpholinosydomine (SIN-1), which releases O2(-*) and NO simultaneously, with or without pre-treatment either with hemoglobin (3 microM) or melatonin (0.1-10 microM). SIN-1 produced a significant dose-dependent relaxation of vascular tension. Pre-treatment with hemoglobin did not affect SIN-1-induced relaxation. Melatonin significantly reduced the vasorelaxant effect of SIN-1 in a concentration-dependent manner. These findings indicate that ONOO attenuates vascular tension in the human umbilical artery. Melatonin significantly suppressed the vasorelaxant effect of SIN-1, possibly due to its ability to scavenge ONOO-.