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On 20 young men and 15 young women, immersed up to the neck in warm water, the pulmonary residual volume was measured by an open-circuit nitrogen-dilution method and by a closed-circuit method. The differences were not significant and there was no significant difference when the measurements were repeated on some of the men in air by the closed-circuit method. The closed-circuit method is quicker and, in this investigation, proved more reliable. When body fat is estimated from body density by the method of underwater weighing, the error introduced by assuming a mean pulmonary residual volume for young men and for young women is considerably greater than the error due to errors in measurement of the pulmonary residual volume. It follows that this parameter should be measured in every case.