Effects of hemorrhage and reinjection on the acidity of the arterial and venous blood were studied with the manganese dioxide, quinhydrone and hydrogen electrodes. Room air was administered by normal and artificial ventilation, by the closed circuit method. Pulmonary ventilation and changes in oxygen consumption were recorded.Hemorrhage elicited an increased alkalinity of the arterial blood, and an increased acidity of the venous blood. The increased alkalinity of the arterial blood was greater than the increased acidity of the venous blood. Subsequent injection of the blood decreased the alkalinity of the arterial blood, and the acidity of the venous blood. The extent of these changes varied with the animal and with the amount of hemorrhage. The decrease in acidity of the venous blood on reinjection may be preceded by a temporary increase in acidity. The acidity may not return to normal. On reinjection, the arterial blood may turn more acid than normal, and fail to reach the normal acid value during recovery. The acidity changes occurring during hemorrhage were accompanied by decreased oxygen consumption and increased pulmonary ventilation, and re-injection was accompanied by reverse changes.Granting an increased oxidation of hemoglobin during hemorrhage, the increased alkalinity of the arterial blood seems related to increased pulmonary ventilation. and an increased blowing off of carbon dioxide.Hemorrhage during constant artificial ventilation elicited similar directional changes in acidity of the arterial and venous blood. These arterial changes in acidity might conceivably be explained by the improved ventilation, due to a decreased flow of blood through the lungs. This being true, the significance of volume-flow of blood through the respiratory center is supported.