A free radical gas, nitric oxide, has many useful functions when produced under physiological conditions by neurons and endothelial cells. However, excess nitric oxide has been reported to exert cytotoxic effects by direct toxicity or by reaction with superoxide. The effect of nitric oxide on the microcirculation in the periphery of a flap remains unclear, and its effect on flap survival is also unknown because nitric oxide has a dual action. Thus, we attempted to clarify the effect of nitric oxide on survival of rat random pattern skin flaps by the use of an endothelial constitutive nitric oxide synthase inhibitor (i.p. administration of 50 mg/kg N(G)-nitro-L-arginine) and the substrate of nitric oxide synthase (i.p. administration of 1 g/kg L-arginine). Three kinds of experiments were done using a total number of 120 animals: (1) time course measurement of blood flow in the flap periphery was performed using a laser Doppler flowmeter (30 rats), (2) the length of the surviving area of flaps was measured 1 week after raising the flap (60 rats), and (3) Western blot analysis was used to determine the time course of changes in the amount of endothelial constitutive nitric oxide synthase and the formation of 3-nitro-L-tyrosine, which is a marker of peroxynitrite-mediated (i.e., nitric oxide-dependent) tissue damage (30 rats). Inhibition of endothelial constitutive nitric oxide synthase by N(G)-nitro-L-arginine significantly decreased the length of the surviving area of skin flap (p < 0.01 compared with the control), which was associated with a decrease in the blood flow of the proximal portion of the flap. On the other hand, exogenous L-arginine increased the survival length of skin flap significantly (p < 0.01 compared with the control), which was associated with an increase in blood flow of the distal portion of the flap even though there was nitric oxide-mediated oxidative tissue damage. These results suggest that nitric oxide produced by endothelial constitutive nitric oxide synthase plays a role in maintaining circulation in the skin flap periphery and that L-arginine administration contributes to reduction of ischemic necrosis in the skin flap.