The experiments with albino rats, Wistar rats and SHK mice showed that the combined use of amphotericin B and diacarb markedly potentiated the effect of the latter: the amount of the urine excreted within 6 h and the renal excretion of sodium ions increased 1.8 and 1.5 times, respectively. Analogous trends were observed in the experiments with stabilization of the animal water balance by 2 per cent water loading. The studies on rats with Pliss lymphosarcoma showed that the diuresis level in the tumor-bearing animals was 3 times lower than that in healthy rats. The treatment with 5-fluorouracil lowered this index more significantly. With the use of amphotericin B, diacarb and their combination the urination level came to normal. The cyclophosphamide therapy of mice with sarcoma 180 was accompanied by accumulation of water in the tumor tissue. The combined use of the cytostatic with the polyenic antibiotic and diuretic resulted in a significant lowering of this index. In rats with Walker-256 carcinosarcoma, the treatment with the above three drugs lowered the sodium ion concentration and increased the calcium ion concentration in the tumor tissue. At the same time the electrolyte level in some organs not affected with malignant tumor (spleen, thymus, liver and adrenal glands) returned to normal.