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Functional residual capacities were measured with the helium-dilution technique in conscious subjects breathing air and then breathing 100 per cent oxygen. In seated subjects there was no change in FRC In the supine position there was a small (3.3 per cent) decrease in FRC on breath-ins oxygen. FRC values of 11 patients were measured while they were awake and breathing room air and then during anesthesia while they breathed halothane in either 30 per cent or 100 per cent oxygen. There were consistent decreases in FRC following induction, not progressive with time and not related to the inspired mixture. The decreases in FRC correlated with the ratio of weight (kg) to height (cm), such that FRC after induction, expressed as percentage of control FRC, = 137.7 −164.4 weight (kg)/height (cm) There was no evidence that the decreases in FRC were due to increased expiratory muscle tone. Spontaneous big breaths during anesthesia restored FRC only partly. A possible mechanism for the decrease in FRC during anesthesia in the supine position is airway closure at low lung volumes due to relative obesity and inadequacy of the sigh mechanism.