The favourable effect of gentamicin and its combination with furosemid was shown in treatment of rats with experimental pyelonephritis. However, alongside the favourable effect, a danger of the gentamicin nephrotoxic effect, especially in combination with furosemid was noted. The nephrotoxic effect was evident from foci of distrophic and necrobiotic changes in the epithelium of the convoluted tubules, impairment of the cortical hemodynamics and development of the cortical hypoxia of the kidneys resulting in severe renal insufficiency. Gentamicin had no direct inhibitory effect on the tissue respiration, did not block the oxygen uptake and oxidative phosphorilation in isolated mitochondria. To prevent the development of the nephrotoxic effect of gentamicin and its combination with furosemid strict and effective control of the antibiotic plasma levels is necessary. Informative tests for the control of the renal function are the concentration parameters of creatinine and urea, especially at the beginning of the pathological state when the level of hyperazotemia is still of a low informative value. The diurnal urine excretion is not an important informative index of renal function.