Nitric oxide is a small gaseous signaling molecule which functions in the regulation of plant development and responses to biotic and abiotic stresses. Recently, we have shown that nitric oxide is required for development of functional nodules. Here, we show that inhibition of nitric oxide synthase enzymatic activity (using N(ω)-nitro-L-arginine) reduces nitric oxide content in soybean root nodules and this is coupled by reduction of endogenous cyclic guanosine monophosphate content in the nodules. We postulate that the regulation of soybean nodule development by nitric oxide is transduced via cyclic guanosine monophosphate through activation of nitric oxide-responsive soluble guanylate cyclase. Furthermore, we hypothesize that this signaling cascade is mediated via modulation of the activities of antioxidant metabolic pathways.