Chemical oxidation of N-hydroxy-L-arginine (NOHA) and other N-hydroxyguanidines has been previously shown to generate either nitric oxide (NO) or nitroxyl (HNO), depending on the oxidative conditions. Because N-hydroxy-L-arginine has been demonstrated to be a biosynthetic intermediate in the oxidative conversion of arginine to endothelium-derived relaxing factor, the possible formation of HNO through a biological process was considered. This study, therefore, explores the biological activity of HNO as a possible effector molecule, and the results indicate that HNO is capable of eliciting vasorelaxation in both rabbit aorta and bovine intrapulmonary artery by a guanylate cyclase-dependent pathway. The pharmacological properties of HNO were very similar to those of endothelium-derived relaxing factor, and the possible relationship between HNO and endothelium-derived relaxing factor is discussed.