17 alpha-ethynylestradiol (EE2), a synthetic estrogen used in birth control pills, has frequently been detected in aquatic environments. In the present study, male and female individuals (3+ ages) of Chalcalburnus tarichi, an endemic cyprinid fish living in the Lake Van basin located in the eastern Anatolia region of Turkey, were exposed to nominal concentrations of 1, 10, and 100 ng/L EE2 for 32 days at semistatic daily renewal conditions. After exposure, plasma vitellogenin (Vtg), hepatosomatic index (HSI), and liver glutathione-S-transferase (GST) activity were examined. Plasma Vtg concentrations exhibited significant inductions in all doses of EE2 in both sexes. HSI did not change at the 1 and 10 ng/L EE2 concentrations; it, however, significantly increased after 100 ng/EE2 exposure in males and females. Liver GST activity significantly increased in females at the 1 ng/L EE2 exposure, whereas it did not change in males. On the other hand, GST activity was inhibited in both sexes at the 10 and 100 ng/L EE2 exposures. The results of this study indicate that C tarichi is highly suspectible to EE2 and its detoxification capability may be weakened by EE2-inhibiting phase II enzyme, GST.