Unilateral ureteral obstruction (UUO) results in a number of pathophysiological and morphological changes in the renal parenchyma, including interstitial inflammation and fibrosis, apoptotic changes of tubular and interstitial cells. Recent studies have indicated an association between renin-angiotensin system and apoptotic alterations in the kidney after unilateral obstructive nephropathy. In this study, the effect of ACE inhibitors and AT1 receptor antagonists on tubular cell apoptosis and interstitial fibrosis in obstructive nephropathy after UUO in rats was investigated. The study was conducted on Wistar rats with unilaterally ligated ureter and sham operated animals (control group). The rats with UUO were treated with ACE inhibitor (cilazapril) or AT1 receptor antagonists (losartan) and control group was treated with H2O. Sham-operated animals were treated in the same way. Tubular and interstitial cell apoptosis was detected morphologically by hematoxylin and eosin (HE) staining and terminal deoxynucleotidyl transferase-mediated nick end-labeling (TUNEL). The area of intersitial fibrosis was determined using computer-assisted image processing after Gomory silver impregnation of paraffin sections. All experimental animal groups with unilateral ureter ligation showed a significantly increased number of apoptotic tubular and interstitial cells in the obstructed kidney compared with the contralateral, unobstructed kidney. Histomorphometric analysis of renal interstitial fibrotic changes in the groups of rats treated with losartan or water showed a statistically significant difference (p < 0.05) between the operated and sham--operated animals. In conclusion, following UUO there is a significantly increased number of apoptotic tubular cells and interstitial fibrosis in the ipsilateral kidney compared with the contralateral kidney. ACE inhibitors and AT1 receptor antagonists could not decrease the extent of renal cells apoptosis and interstitial fibrosis after UUO.