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Resistance and capacitance of body tissues and blood are reported over a wide frequency range. It is shown that it cannot be assumed in impedance plethysmography that tissue and blood have comparative resistance values and that the capacitance is of negligible influence, if presently used 200 kc. frequency is considered. In order to obtain a stable ratio of blood and tissue impedance, frequency must be below 10 kc.; and in order to exclude capacitive currents, frequency should be near 1 kc. Electrode polarization phenomena and their effect on impedance measuring devices are discussed briefly. It is concluded that not only in two terminal impedance measuring devicess but also in four teminal devicesp as currently used in impedance plethysmography, polarization is disturbing if the frequency is substantially below 1 kc. Therefore, in this article, the conclusion is made that impedance plethysmography should be carried out with frequencies near 1 kc., instead of the currently used 200 kca A mathematical analysis whichrelates quantitatively observed relative resistance change and relative blood volume change concludes this article. This ratio is found near 3.5. It is substantially higher than assumed previously and suggested for experimental verification in order to elevate impedance plethysmography from a qualitative to a valuable quantitative tool for blood-volume research.