This study describes the effects of 17-alpha-ethynylestradiol (EE2) on the structure of the excretory system of the kidney in tench. Adult male tench were exposed to sub-lethal doses of EE2 (50, 100 and 500 μg/kg b.w.) under semistatic conditions for a period of 30 days. The nephrosomatic index and histology (including a morphometric analysis) of the kidney were examined. Histopathological lesions in the kidney of exposed tench were: dilation of glomerular capillaries and increase in the area of the renal corpuscle, hyaline degeneration in the epithelial cells of the proximal tubules leading to necrotic changes, hemorrhages in the interstitial tissue and deposits of eosinophilic material. These lesions were observed with a greater degree of severity as the exposure doses were increased. These results indicate that long-term exposure to EE2 could produce clear negative effects on the excretory system of the kidney in tench and consequently on their physiological functions.