In the experiments performed on 61 dogs dynamics of changes of morphometrical parameters of mitochondria of brush-border and cuboid cells have been studied at conservation of the kidneys for 120 h in solutions of intracellular type. Stages of structural reorganization of the mitochondria have been determined depending on duration of conservation and the solution composition. A morphometrical index of viability (Im) has been worked out, correlated with survivability of the transplant and the recipient. In every specific case it allows to estimate quantitatively whether the conserved kidney is suitable for transplantation. At Im less than 50% from the control level, the kidney is not viable, at 50% less than or equal to Im less than or equal to 60%--it is conditionally suitable for transplantation, at Im greater than 60%--it is viable. The index elaborated is practically used for a comparative estimation of the protective action of the four organ-preserving solutions of the intracellular type. The morphometrical index of viability of the conserved kidneys can be used in experimental transplantology for approbation of new solutions before their application in clinical practice.