Five lambs were anaesthetized, paralysed, mechanically ventilated and connected to a membrane "lung" to permit removal of carbon dioxide. When part of the carbon dioxide was removed in this manner, the tidal volume was decreased to keep PaCO2 constant. For example, when 70% of carbon dioxide was removed by the membrane lung, total ventilation was reduced by 50%, peak inspiratory pressure was decreased by 45%, and PaO2 was kept constant by increasing the inspired oxygen fraction from 0.21 to 0.27%. The removal of carbon dioxide by a membrane during positive pressure breathing could decrease barotrauma, particularly in poorly compliant lungs. Technically, the extracorporeal removal of carbon dioxide is relatively simple procedure.