Studies were carried out on blood phagocytes and alveolar macrophages of 96 humans, on the cells of the viscera and tissue phagocytes (liver, brain, myocardium, lungs, kidneys, stomach, and skeletal muscle), and liver mitochondria of 186 random bred white mice. Generation of the active oxygen forms was determined using different methods after direct effect of CO2 on the cells and biopsies and indirect effect of CO2 on the integral organism. The results obtained suggest that CO2 at a tension close to that observed in the blood (37.0 mm Hg) and high tensions (60 or 146 mm Hg) is a potent inhibitor of generation of the active oxygen forms by the cells and mitochondria of the human and tissues. The mechanism of CO2 effect appears to be realized, partially, through inhibition of the NADPH-oxidase activity. The results are important for deciphering of a paradox of evolution, life preservation upon appearance of oxygen in the atmosphere and succession of anaerobiosis by aerobiosis, and elucidation of some other problems of biology and medicine, as well as analysis of the global bioecological problem, such as ever increasing CO2 content in the atmosphere.