The rate of increase of alveolar concentration of enflurane was studied in patients during general anaesthesia with controlled ventilation. Elimination was studied after 30 and 60 min of body equilibration. Halothane and enflurane were administered simultaneously in a second group of patients to compare their uptake curves. In the same patients the elimination curves of the two anaesthetics were determined after 60 min of body equilibration. In the first few minutes, enflurane uptake was more rapid than that of halothane. After 15 min the two curves were similar. Multi-compartment analysis of the uptake and elimination curves of the two anaesthetics showed a large difference in the slope of the slowest compartment. This could be accounted for by the lower fat solubility of enflurane compared with that of halothane.