L-arginine-dependent synthesis of nitrite (NO2-) and nitrate (NO3-) by macrophages correlates with and is required for their execution of nonspecific cytotoxicity toward some tumor cells and microbes. However, the bioactive L-arginine metabolites responsible for cytotoxicity are unknown. Mammalian endothelial cells have recently been shown to release nitric oxide (NO.); we therefore determined if this reactive metabolite was synthesized by activated murine macrophages. Macrophage-derived NO. was detected by two independent methods: a bioassay for NO.-mediated relaxation of preconstricted rings of rabbit aorta; and a spectroscopic measurement of the reaction of NO. with clostridial ferredoxin, an Fe-S protein. After activation with IFN-gamma and LPS, macrophages continuously secreted a substance that relaxed rabbit aortic rings denuded of endothelium. Production of the vasorelaxant was enhanced by 0.5 mM L-arginine and inhibited reversibly by NG-methylated L-arginine analogs that block macrophage NO2-/NO3- synthesis. The vasorelaxant was scavenged by ferrous myoglobin, was labile, and was neither NO2- nor a cyclooxygenase metabolite. Activated M phi also secreted a substance that bleached Fd, a reaction carried out by NO. and NO2, but not NO2-. Macrophage bleaching of Fd correlated directly with time, cell number, and concomitant NO2-/NO3- production, required L-arginine, and was independent of reactive oxygen intermediates. Thus, activated murine M phi release NO. and/or a closely related, highly reactive nitrogen oxide such as NO2, during their conversion of L-arginine to NO2-/NO3-. NO. and NO2 may mediate L-arginine-dependent pathologic effects of M phi, as well as physiologic effects not previously considered for this widely distributed cell type.