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Relaxation of blood vessel walls is one of the physiological effects induced by visible radiation. This effect is similar to the endothelium-derived relaxation (based on endothelium-derived relaxing factor, EDRF) and depends on the presence of nitric oxide donors. We assumed that nitric oxide hemoglobin (HbNO) might participate in the mechanism of this phenomenon and investigated the photochemical reactions using low-power He-Cd (441.6 nm) and He-Ne (632.8 nm) lasers. It was found that in the presence of oxygen the He-Cd laser but not He-Ne laser induced a photolytic decrease of the HbNO EPR signal amplitude by 25-35%. Using optical absorption measurements, we showed that methemoglobin was formed during irradiation. The comparison of the kinetics of this process and the irreversible reduction of paramagnetic centers suggest that the laser radiation leads to dissociation of the heme-NO complexes, but not to oxidation of the heme iron. In the absence of oxygen the photodissociation effect did not exceed 5-10% and was reversible. We suppose that this finding can be explained by fast reassociation of NO with unligated Hb. The EPR spectrum of iminonitroxide was observed upon irradiation of HbNo with nitronyl nitroxide in a vacuum, suggesting the formation of free NO. Thus, our experiments confirm that NO-hemoglobin can reversibly dissociate and release free NO during exposure to He-Cd laser radiation.