The in vivo estrogenic activity of the two branched alkylphenols, tert-octylphenol and technical nonylphenol, and the two linear isoforms, n-octylphenol and n-nonylphenol, was compared. The compounds were administered to juvenile rainbow trout (Oncorhynchus mykiss) by either intraperitoneal injection or water exposure and their estrogenic potential was evaluated by ELISA measurements of induced plasma vitellogenin. Intraperitoneal injections (50 mg/kg) of the two branched alkylphenols resulted in a significant vitellogenic response after 12 days whereas no significant induction was seen with the two linear isomers. Water exposure for 9 days to a nominal concentration of 150 micrograms/l of the alkylphenols elicited the same response pattern as seen for the injection experiment. Furthermore, in the present vitellogenin assay tert-octylphenol was giving a higher estrogenic response compared to technical nonylphenol using either of the two exposure routes.