Cardiac output was measured in dogs before, during, and after intravenous infusion of gaseous CO2. Slight decreases in output occurred but not great enough to indicate a significant impairment in blood flow through the pulmonary system. Infused CO2 moves rapidly into the expired gas. Lowered oxygen saturation of blood following infusion of CO2 can be explained in part by the Bohr effect. However, this does not account entirely for the level of oxygen unsaturation and something more than the Bohr effect seems involved.