Anesthetized rats breathed during 1 hr with gas mixtures containing 16 to 6 per cent of oxygen. At 6 per cent of oxygen, the hypoxia of pulmonary tissue and its processes of peroxide oxidation of lipids entails a sharp increase in elastic resistance against breathing. This is combined with insignificant disorders in tidal volume which is not compensated by an increase in the breathing rate. General mechanisms of external respiration disorders in various types of hypoxia and in stress, were revealed.