1. Curves showing the rate of oxygen consumption in normal rabbit blood, defibrinated in the air, are given. 2. Among the blood specimens washed with various physiological solutions, that washed with Ringer's solution seems to be the most natural, the one washed with isotonic glucose solution the next, the one washed with saline solution the most unnatural. 3. Sodium, potassium, magnesium and calcium ions have, in M/100 concentration, an effect augmenting the oxygen consumption of blood, its intensity increasing in this order, proportionally to the atomic weight. But the duration of the augmentation diminishes in inverse proportion to its intensity. All four ions show, in the concentration of M/10, an evident inhibitory effect, whose intensity varies in the same order as before. 4. Between potassium (resp. sodium) and calcium (resp. magnesium) ions an antagonism is recognizable, and a slight synergistic effect is observed between potassium and sodium ions, on one side, and between calcium and magnesium ions, on the other. 5. The hydrogen ion concentration of rabbit blood, defibrinated and fully oxygenated in the air, amounts at room temperature between 15°C. and 20°C. to pH=7.55-8.09 in eleven cases. 6. Sodium, potassium, magnesium and calcium ions increase the alkalinity, in other words, diminish the hydrogen ion concentration, of blood in this order. The intensity of the effect is proportional to the concentration. When any two of them are applied in combination, the alkalinity of the blood increases more intensely than in the case of one alone. In no case is a counterbalancing effect recognizable, as observed in the case of the reduction rate of blood. 7. The antagonism between potassium (resp. sodium) and calcium (resp. magnesium) ions observed from their effect upon the gas metabolism of blood can not be explained merely by the alteration in the hydrogen ion concentration of blood; in other words, the antagonism of these ions revealed in their biological action is not simply on account of the physico-chemical changes brought about by them. One may conclude that this antagonism is to be ascribed to an unknown character peculiar to each ion. I wish to express my obligation to my teacher Professor K. Miurain Tokyo, who has sent me to this Laboratory for the investigation of blood gases, and to Dr. K. Inouye, Professor of Biochemistry in Sendai, who has had the kindness to let me continue my work in his Laboratory in determining hydrogen ion concentration, following the fire which broke out in this Laboratory early this year.