1. Physiological dead-space volume (VD) was measured in twenty-four healthy men and women aged from 20 to 71 years, at rest and at two rates of work on a treadmill, whilst breathing air and breathing oxygen.

2. The effect of correction of arterial carbon dioxide tension (Pa,co2) to pulmonary capillary temperature on the resulting value for VD was investigated. We find that the effect is substantial and that a correction should be made.

3. Equations have been derived for the prediction of normal VD during exercise. The best prediction was given by a regression on height, age, carbon dioxide output, ventilation and respiratory frequency, with an upper 95% confidence limit of +81 ml.

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