Summary Eight out of eleven dogs were long term survivors when submitted to 24 hours of partial heart-lung bypass using a membrane oxygenator with a low priming volume. It was possible to maintain an adequate metabolic balance in these animals and tests of hepatic and renal function after perfusion gave normal results. Blood damage during perfusion remained well within limits of compensation by the organism. However, the survival time of erythrocytes tagged at the end of the procedure was shortened, explaining the occurrence of hemolytic anemia in the first week after perfusion. Nonsurvivors showed pulmonary damage, water and electrolyte retention, splanchnic congestion and impairment of liver function. These experiments suggest that extracorporeal blood oxygenation can be offered safely for clinically useful periods with simple equipment requiring little or no priming blood.