Low Pco2 in the ambient water owing to diffusion of expired carbon dioxide into nitrogen bubbles, in the present experiments, reduced Pco2 in the arterial blood, increased oxygen affnity of the blood, and caused lowering from 70-80 mmHg to 30mmHg of the minimnum level of ambient oxygen required for maintenance of oxygen concentration in the arterial blood. The higher value obtained by the previous experiments, where accumulation of carbon dioxide in both water and blood was not avoided, is considered to be reliable as the criterion of ambient oxygen, because decrease of ambient oxygen is usually followed by increase of ambient carbon dioxide at an equal ratio. The present value is considered to be useful for discussion in basic physiology of carp respiration, because the effects of hypoxia on fish are clearly shown by avoiding the effects of carbon dioxide accumulation.