An experimental study on disturbances of acid-base balance encountered during operations of opened chest was carried out in dogs and rabbits. Special attention was given to the effects of methods for continuos O2 supply under positive pressure. The following results were obtained: 1. When bilateral thoracotomy is performed with constant flowing of O2 under positive pressure, the removal of CO2 is hindered, the respiratory acidosis sets in with the lapse of time and the danger ensues. In fact, in spite of enough supply of O2, the animal dies due to the acidosis. 2. The respiratory acidosis during operation will be relieved by closing the chest.3. The respiratory acidosis during operation is often complicated by metabolic acidosis.4. Metabolic acidosis appears few days after operation and it tends to increase. Concerning the research how to prevent the occurrence of acidosis during operation the following results were obtained:5. When the intermittent administration of O2 under positive pressure or the constant flowing under positive pressure by intermittent insufflation of xcessive O2 is used to expedite the exchange of the alveolar gas, the respiratory acidosis is prevented.6. The partial pressure of CO2 is not increased by the type of intermittent administration of O2 under positive pressure, but the alkali reserve decreases and the pH falls.7. Metabolic acidosis as well as respiratory acidosis do not occur by the type of constant flowing under positive pressure by intermittent insufflation of excessive O2. More physiologic, accordingly more suitable condition can be obtained by this means than by the above mentioned method. Kind advices were given by Prof. K. Matsuda, of applied physiology I should like to express my gratitude to him.