Once plant endogenous nitric oxide (NO) production had been proved, NO research was directed toward both the source and the targets of this extremely bioactive molecule. As in mammals, plant NO was first thought to be generated mainly by a NO synthase-like enzymatic activity. However, nitrate reductase (NR)-dependent NO production is now receiving much of the attention because of the ubiquity of this enzyme in higher plant tissues and the precise regulation of its NO-production activity. NO has been reported to be a signal in many and diverse physiological processes, such as growth and biotic and abiotic stresses. Recently, NO has been shown to affect stomatal closure and interact with abscisic acid signaling pathways. We propose NR as a putative component in the signaling cascade of ABA-induced stomatal closure.