The respiratory dead space in Japanese healthy 45 males and 30 females were estimated by Bohr formula from the tidal volume and the CO(2) percenttage of alveolar and expired air in sitting posture during rest. The relationship between the respiratory dead space and other physical or respiratory data were analysed statistically. The mean value of the dead space was 138±21ml. for males and 109±14ml. for females. in Japan. And the ratio of dead space to tidal volume averaged 31 percentage for both males and females. The respiratory dead space showed a definite correlation with individual vital capacity and tidal volume.