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Previous studies have noted effects of pulp and paper effluents on the physiology of fishes including; smaller gonads, increased age to maturation, alterations in secondary sex characteristics, reduced plasma sex steroid levels, and the induction of vitellogenin (Vtg) in males and juveniles. A program to determine the potential impacts of a modem New Zealand pulp and paper mill effluent on fishes employed a combination of long- and short-term exposures of the juvenile (1+) rainbow trout, Oncorhynchus mykiss, to the effluent. Juvenile (1 + aged) rainbow trout were exposed to a mixed thermomechanical pulp/bleached kraft (TMP/BK) mill effluent at a range of concentrations from environmentally relevant (10%) to 70% (effluent by volume) in three exposure studies. During 21-,56- and 320-day exposures to 10% and 30% (v/v) effluent, no significant impacts on circulating testosterone, and pregnenolone levels were observed. No significant induction of liver MFO activity was observed at any exposure concentration. Vitellogenin induction or expression of the estrogen receptor in juvenile males was not observed in fish from either experiment. High experimental mortality was observed in fish exposed to 70% (v/v) secondary treated effluent compared to a reference treatment during the 2l-d exposure and was linked to an atypically high suspended solids load. Thus, the combined data from these experiments demonstrated a lack of estrogenicity or impacts on steroidogenesis following exposure to TMP/BK mill effluent. However, a parallel experiment using sexually mature rainbow trout did show elevated Vtg in males. Thus, the presence of estrogenic compounds cannot be ruled out, but appears to be infrequent. Results from parallel studies with mosquitofish have shown androgenic activity in the effluent under investigation.