The aim of this research was to specify M. I. Tischenko's formula (1978) for the estimation of the volume of extracellular liquid on the basis of integral resistance so as to use it in children. Simultaneous assessment of the volume of extracellular fluid by thiosulfate dilution method and registration of the basal integral resistance with data processing helped correct the formula for use in practical pediatrics. The technique of assessment of the volume of extracellular fluid is simple, does not involve intravenous infusions of drugs or blood collection, rapid, and reproducible under any conditions. It may be used for assessment of the volume of extracellular fluid in children with the edematous syndrome associated with renal diseases (acute renal failure, glomerulonephritis), liver diseases, circulatory insufficiency. Repeated studies give valuable information about the state of water-salt metabolism in patients with hyper- or dehydration and help assess the therapy efficacy.