1. The point of onset of ‘airway closure’ has been determined by simultaneous analysis of expired argon and nitrogen in twenty-five subjects studied in the sitting and supine postures.

2. Estimates of ‘closing volume’ derived from nitrogen analysis were less than those derived from argon analysis, the difference being partly attributable to a less readily defined inflexion in the record of expired nitrogen.

3. The relationship between the two estimates of ‘closing volume’ was linear and was not influenced by the age or posture of the subjects.

4. It is valid to use nitrogen as the tracer gas for the measurement of ‘airway closure’, despite the difference in the formation of the inspired bolus, but results should be compared with normal values established separately for this gas.

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