Continuous pressure breathing was studied in hypothermic anesthetized dogs. Alveolar ventilation decreased during continuous positive pressure breathing (CPPB) and increased during continuous negative pressure breathing (CNPB). The changes in alveolar ventilation were due to changes in respiratory rate as well as in respiratory dead space. Cardiac output fell significantly during CPPB due to a reduction in heart rate and stroke volume. During CNPB cardiac output was only slightly greater than during control as a result of a fall in heart rate and an increase in stroke volume. Oxygen consumption was reduced to 60% of control during CPPB of 16 cm H2O, but was 25% greater than control during CNPB. Qualitatively, CO2 production changed as did O2 consumption, but was different quantitatively during CNPB, indicating hyperventilation due to increased respiratory rate. Mean pulmonary artery pressures and pulmonary resistance varied directly with the applied intratracheal pressure. The results indicate that the hypothermic animal can tolerate an imposed stress such as CPPB and can increase its O2 consumption during CNPB as does the normothermic animal. (Author).