I examined the effects of a modern mixed thermo-mechanical pulp/bleached kraft (TMP/BK) mill effluent on potential reproductive-endocrine impacts on two fish species, rainbow trout and mosquitofish using a combined approach of laboratory and on-site mesocosm exposures. Rainbow trout Oncorhynchus mykiss were utilised in a combination of early life stage and juvenile (+1 year) long- and short term exposures. Exposure of rainbow trout eggs to 15% (v/v) effluent until 10.5 months of age had no effect on fertilisation, hatching success, time to hatch, time to swim-up, juvenile mortality rate, growth and development. Juvenile (+1 year) rainbow trout were exposed to secondary treated TMP /BK mill effluent at a range of concentrations from environmentally relevant (10%) to 70% (v/v) in two exposure studies. During both 21- and 56-day exposures to 10% and 30% (v/v) effluent, no statistically significant impacts on spleen weight, liver weight, condition factor, and circulating testosterone and pregnenolone levels were observed. Consequently, vitellogenin induction as well as the expression of the estrogen receptor in juvenile males was not observed in either experiment. Statistically significant differences in spleen and liver size as well as high experimental mortality were observed in the 70% (v/v) effluent 21-day treatment and was linked to an atypically high suspended solids load. Two mosquitofish Gambusia affinis experiments were conducted, both consisted of a 21-day exposure using adult females. The first experiment was a multiple concentration exposure to primary and secondary treated effluents. A statistically significant masculinisation response (gonopodial development) was noted in all effluent treatments. However, secondary treatment of the effluent resulted in a significant decrease in gonopodial development. The second experiment focused on filtered and unfiltered secondary treated effluent at an environmentally relevant concentration (15%). Filtration of the treated effluent resulted in the almost complete elimination of the female masculinisation response. In all experiments, male mating behaviour was observed in the masculinised female mosquitofish. I concluded that a significant species difference regarding sensitivity to reproductive-endocrine modulating compounds occurred following exposure to a mixed TMP/BKM effluent. In addition, the data suggest that the compounds of concern are bound to solid matter within the effluent and these effluents have the potential to exert effects through an androgen-like mode of action.