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A nitrogen-dilution technique for measurement of airway closing volumes (CV) and functional residual capacity (FRC) not requiring subject cooperation was tested in five healthy, awake, spontaneously breathing subjects and subsequently used in 20 patients during anesthesia with mechanical ventilation. Incomplete exhalation before inhalation of oxygen did not significantly alter CV. Inspiration of a volume of oxygen equal to 75 per cent of vital capacity (VC) did not affect CV, whereas inspiration to 50 percent VC resulted in a 20 per cent decrease in CV. Expiratory resistance tended to reduce CV. By means of this technique, the validity of which had been thus demonstrated, airway closure could be shown to occur at lung volumes larger than FRC (and thus within a normal tidal volume) in six patients prior to anesthesia, and in a further II (total 17 of 20) during anesthesia with mechanical ventilation. FRC decreased by an average of 0.5 liters during anesthesia with mechanical ventilation and was only 0,2 liters above residual volume. Significant hypoxemia was observed in association with airway closure.