17-beta-Estradiol is naturally converted in numerous organisms to various derivatives/metabolites, which may be excreted from the organism into its immediate external environment. There is a paucity of data regarding the biological effects of these derivatives/metabolites on aquatic organisms. Male channel catfish (200-500 g, N=5, 12-18 months) were injected with 1 mg/kg 17-beta-estradiol (E2), ethynyl estradiol (EE2), estrone, estriol or E2-17-beta-glucuronide with subsequent measurements of vitellogenin (Vtg) and serum E2 concentrations 7 days post injection. EE2 and E2 gave the largest magnitude of Vtg response followed by estrone and estriol. Exposure to EE2, estrone, and E2-17-beta-glucuronide all induced significant increases in serum E2 concentrations. This study indicates that metabolites of E2 are also estrogenic and may potentially disrupt estrogen feedback loops within aquatic organisms.