The occurrence of certain natural and synthetic steroidal estrogens in the final effluent from STW has been demonstrated. 17β-Estradiol and estrone were present at concentrations in the tens of nanograms per liter range, and the synthetic estrogen 17α-ethynylestradiol was also identified, albeit in the low nanogram per liter range. The findings from subsequent in vivo tank trial experiments, in which adult male rainbow trout (Oncorhynchus mykiss) and adult roach (Rutilus rutilus) were exposed for 21 days via the water to environmentally relevant concentrations of 17β-estradiol and estrone are presented. In addition, the response of adult male and female roach following exposure to 17β-estradiol (1, 10, and 100 ng/L) was compared to the response to the alkylphenolic xenoestrogen, 4-tert-octylphenol (1, 10 and 100 μg/L). Plasma levels of vitellogenin were determined using previously validated radioimmunoassays in order to measure the estrogenic response of the fish to the varying concentrations of the compo...