Changes in cardiorespiratory function resulting from haemorrhage and overtransfusion were measured in forty-seven dogs during controlled ventilation. In the animals bled to a mean arterial pressure of 70 mm Hg there were significant reductions in arterial Pco2, carbon dioxide output and oxygen consumption, cardiac output and total venous admixture. In the animals transfused to a mean central venous pressure of 15 mm Hg there was a significant fall in total thoracic compliance and arterial Po3 and significant increases in the alveolar-arterial Po3, difference and carbon dioxide output.