The idea of this paper is to examine, after a discussion of the theoretical foundations, to what extent impedance plethysmography enables us to measure changes in the volume of the human extremities caused by capillary fluid filtration. Possibilities of error shown up in this paper - e. g., resistance of the skin, phase shift between current and voltage, the influence of specific resistance, dependence on temperature - partially confine the applicability of impedance plethysmography to comparative relative measurements. In these cases and in full view of the possibilities of error this method offers the advantage of absolutely pressureless measurement and the opportunity to extend this procedure to days.