Ten normal adult dogs were subjected twice to the acute loss of 7-10 cc/kg blood at the intervals of 1 hour and killed 1 hour later and pathological changes in the respiratocirculatory system and lungs were investigated. The results were as follow: 1) Although the pulse became rapid and weak, and the arterial pressure fell abruptly after the acute blood loss, they gradually tended to recover, but not quite up to the normal until killing. 2) The respiratory rate and tidal volume markedly increased, but oxygen consumption gradually decreased. These findings suggested the occurrence of changes disturbing the effective ventilation in the lungs. 3) Electrocardiographically, the P-pulmonale by the increased resistance of pulmonary circulation were observed in few dogs. 4) Microscopically, congestion and hemorrhages in the lungs and the partial breakdown of alveolar structures were observed, but pulmonary edema was never found. 5) Pulmonary edema could not be caused by the acute blood loss mentioned, but the preparatory conditions of it were considered, regarding the increased resistence of pulmonary circulation, anoxia and the increase of the blood volume of the lungs.