The helium dilution technique has been used to measure the FRC of 13 patients before and during anaesthesia with paralysis and artificial ventilation, and also in 5 conscious subjects during spontaneous and artificial ventilation without paralysis. After induction of anaesthesia the mean reduction in FRC was 297 ml or 15.4% of the preoperative value and the change was highly significant (P is smaller than 0.005). In the conscious subjects, the FRC was reduced in every case during artificial ventilation by a mean value of 99 ml or 4.1% of original value. This change was also significant. The anaesthetized patients breathed 35% oxygen before and during anaesthesia. The (A-a) PO2 difference increased from a mean value of 49.1 mm Hg before anaesthesia to a mean of 79.7 mm Hg during anaesthesia, the change being highly significant (P is smaller than 0.001). Individual changes correlated with the reduction in FRC (P is smaller than 0.005).