as part of their manufacturing process, pulp and paper mills release effluent into waterways that may affect the fecundity, morphology, and physiology of invertebrates and vertebrates in the receiving ecosystem (kovacs et al. 2006, mcmaster et al. 2003, munkittrick et al. 1998). treatment systems within the pulp and paper mills are effective at removing many toxicants and improving effluent quality; however, pulp and paper mill effluents may still negatively impact the aquatic environment. fathead minnows (pimephales promelas) are a useful vertebrate model species for environmental monitoring because they spawn frequently, reproduction can easily be monitored, and a significant quantity of data has been published regarding their responses to chemicals and effluents (rickwood and dube 2007, kovacs et al. 2005, parrott and wood 2004, among numerous others). our research involved using a short-term fathead minnow reproductive bioassay (which includes a 15-day pre-exposure period and a 6-day exposure period) in order to assess consistency and predictability of spawning, determine reproductive and physiological changes resulting from exposure to 10% (v/v) untreated kraft mill effluent (uk), 25% (v/v) secondary treated kraft mill effluent (sk), and 100% (v/v) combined mill outfall (cmo), and analyze the applicability and relevance of our 6-day reproduction test. two set of experiments were run: river water vs. kraft mill effluent, and river water vs. combined mill outfall (cmo) effluent. pre-exposure and control fish showed predictable spawning, although a number of breeding pairs were required in order to ensure a sufficient sample size. ten percent (v/v) uk decreased egg production dramatically, and 10% (v/v) uk and 25% (v/v) sk each caused masculinization in a female fathead minnow. while clarification and secondary effluent treatment appeared to improve the short-term reproductive impacts on fathead minnows observed in kraft mill effluent, these processes did not entirely remove the source of endocrine disruption causing masculinization. in contrast, the 100% (v/v) cmo effluent did not cause any observable reproductive or physiological changes. a short-term (6-day) exposure period appears to be sufficient for analysis of the effect of edcs on vertebrate morphology and fecundity, although it is unclear whether responses were not observed in sk and cmo effluents because of the short observation period. study site : abitibibowater mill in thunder bay, northwestern ontario (discharge into the kaministiquia river).