Tritium-labeled water was injected intravenously into a young male adult and the time courses of specific radioactivity in plasma, red cells, urine and expired water vapor were determined and compared. Specific activity of breath water and urine appeared to follow a pattern of damped oscillation about the curve for the specific activity of plasma water. The transfer function of the biological system for the injection of radioactivity was determined. Results indicated that breath water samples may under certain circumstances be substituted for urine or plasma water samples in the determination of total body water.