Chronic venous insufficiency is a frequent health problem, especially in females, with a chronic evolution and important social costs. The study aims to identify the sources of reactive oxygen species in chronic venous insufficiency, especially the NADPH and NADH mitochondrial oxidases, by way of a histochemical technique employing nitro blue tetrazolium (NBT). This method allows for the evaluation of the oxidative enzyme activity in the tissue due to NBT’s property of being eliminated by the tissue dehydrogenases, with the formation of a blue non-crystalline pigment, called formazan. For this purpose the fragments of varicose veins were taken from 31 patients diagnosed with CVI stage II/III (class CEAP 2-5) with an average age of 46.2 during a surgical procedure (venous stripping). The results were compared to those of a control group of the same sex and age distribution, taken from normal saphenous vein during the surgical procedures of coronary and peripheral by-pass. The CVI diagnosis was clinic (the presence of varicose veins, shank oedema, lipodermatosclerosis, varicose eczema and post venous ulceration scars). The arterial involvement was excluded through peripheral Doppler probe. In chronic venous insufficiency there is an intense mitochondrial oxidant enzyme activity, which intervenes in the production of superoxide anion and indirectly of oxygenated water. These enzymes are active in the whole wall of varicose vein, but mostly in the hypertrophycal muscular layer and conjunctive layer where rich fibroblasts can be seen. The activity of NADPH oxidase is high in the luminal thrombus possibly in neutrophils and fibroblasts infiltrate. The histochemical quantification of the oxidative state of the tissue by means of using NBT proved to be an efficient method of demonstrating the excess production of reactive oxygen species and of analysing the antioxidant state of the tissue.