Comparison of electrical impedance measured in vivo with the in vitro impedance of several types of tissue shows that the former is mainly determined by the presence of striated muscle in the region between the electrodes. This means that fluid shifts occurring during hemodialysis, in the blood or any tissue other than skeletal muscle, do not affect tissue impedance. This implies that measurement of tissue impedance as a means of monitoring the process of hemodialysis lacks a model for a reliable interpretation in terms of fluid balance.