To help decide when an inhalational agent should be discontinued during cardiopulmonary bypass (CPB), its rate of washin and washout must be known. Isoflurane one per cent was administered to 14 patients undergoing CPB and isoflurane blood concentrations were measured to determine the time course of washin and washout of this agent. Bubble oxygenators were used for seven patients and membrane oxygenators for the remaining seven. During the administration of isoflurane, isoflurane blood concentrations rose slowly and did not reach a steady state during the time available for washin. Isoflurane blood concentrations decreased by at least 50 per cent within two minutes of turning off the vaporizer, and by 15 minutes the concentration had dropped by 75 per cent. There was a tendency for more rapid elimination of isoflurane in patients undergoing rewarming during this period. There did not appear to be an important difference between bubble and membrane oxygenators in the rate of washin and washout of isoflurane. Within 15 minutes of turning off the vaporizer only 25 per cent of the original blood concentration of isoflurane will remain. The anaesthetist must decide what concentration of isoflurane is acceptable during separation from CPB. Knowledge of the time course of isoflurane washout will allow more accurate determination of when to discontinue its administration in order to reach an acceptable concentration by the time separation from CPB occurs.