THE AIM: to estimate the level of expression miRNA-21 inthe urine and renal tissue in rats with unilateral ureteral obstruction (UUO). MATHERIAL AND METHODS. UUO was induced by ligation of the left ureter in male Wistar rats (n=10). Follow-up period was 14 days after UUO modeling. Urine was collected one day before the operation (UmiRNA21C), and one day before the end of experiment (UmiRNA21I ) during 24 hours. Before releasing animal out of experiment collected urine from left kidney pelvis (UmiRNA21O) and tissue of left kidney (KmiRNA21O) and right kidney (KmiRNA21I). MiRNA-21 expression in kidney tissues and urine was carried out with reaction amplification (RealTime PCR-protocol). Calculation was realized by 2-deltaCtmethod. Statistical analysis was performed with Wilcoxon test and Spearman correlation coefficient. Results are demonstrated as median [low – upper quartile]. RESULTS. UmiRNA21I(3.78[2.0-5.28]) and UmiRNA21O(3.78[3.25-3.82]) were significantly higher than UmiRNA21C (1.15[0.71-1.74]; P=0.0125 and P=0.0069, respectively). UmiRNA21I and UmiRNA21O values were practically equal. In kidneys with UUO the tissue levels of miRNA21 expression was a higher than in contralateral organ (P=0,0926). Revealed direct correlation between KmiRNAIand KmiRNAO(RS=0,770, P=0,0092). CONCLUSION. UOO can cause specific changes in the expression, distribution and excretion of micro RNA-21 and its role in the development of renal tubulointestitsial fibrosis requires further studies. Key words: miRNA-21, tubulointerstitial fibrosis, unilateral ureteral obstruction