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An improved plethysmograph is described, which is to be used for the measurement of the body volume of human subjects. Results of measurements of 88 individuals show that the accuracy of the method is similar to that of the underwater weighing method. The difference in under­water weighing volume and plethysmometric volume of 0.21 dm3 at 70 dm3 was significantly different from zero. The estimated standard deviation for the plethysmometric measurement, with assumption that the underwater weighing is without error, is 0.33 dm3. The method is fast, non-invasive and it does not cause any discomfort to the subjects. The conclusion is that the plethysmograph is a valuable and practical instrument for future research work.