Abstract: Apparatus for monitoring body water compartmentalization, specifically intracellular and extracellular body water. The apparatus includes a signal generator for generating a sequence of constant current electrical signals at a predetermined number of frequencies ranging between 1 kHz and 2 MHz. Bioelectric contacts are applied to the subject at two sets of points. The resistance drop across the subject is measured by a resistance measuring circuit using one set of contacts. The phase shift between the signals sensed by the sets of contacts is measured by a phase detecting circuit. The current produced by the signal generator is monitored and regulated to maintain the signals at a constant current. The measured data are combined with anthropometric data entered by means of a keyboard, a scale, or other input device, and processed by means of a microprocessor. The micropressor computes a subject's extracellular, intracellular and total body water as a function of the bioelectric and anthropometric data.