The comparative pharmacology of endothelium-derived relaxing factor (EDRF) and nitric oxide (NO) was studied on isolated strips of rabbit aorta. Both compounds were equally unstable. The relaxations of the bioassay tissues induced by EDRF released by bradykinin (3-100 nM) and by NO (4-134 nM) were indistinguishable. The stability of both compounds was increased to a similar extent by infusions of superoxide dismutase (SOD; 15 U.ml-1) or of cytochrome c (40 microM). The relaxations induced by EDRF and NO were inhibited to similar extents by infusions of Fe2+, hydroquinone and pyrogallol, an effect attenuated by a concomitant infusion of SOD or cytochrome c. The relaxations induced by both compounds were also inhibited by haemoglobin, however, this effect was unaltered by concomitant infusion of SOD. These data indicate that EDRF and NO have identical biological activity, stability and susceptibility to inactivation by superoxide anions and haemoglobin, providing further confirmation that EDRF is NO.