Examination of the environmental effects of effiuent discharges from pulp and paper mills provides an interesting case study in fish toxicology (Figure 24.1). The effects of pulp mill effiuents (PMEs) on aquatic environments have been examined for over 40 years, and extensive studies have been conducted since the mid-1980s. During this period, environmental effects have been observed, regulations have been implemented, and the industry has responded to these regulations, resulting in significant reductions in environmental effects. In the 1970s, many effiuents received no secondary treatment, and environmental concerns were associated with oxygen demand, suspended solids, and nutrient and organic loading (Owens, 1991). These concerns shifted in the mid-1980s when a series of Swedish field studies documented that PMEs induced toxic responses in fish at very low concentrations (Södergren, 1989). Over the next few years, supporting data were obtained in Canada (Hodson et al., 1992; McMaster et al., 1991; Munkittrick et al., 1991, 1992a, b; Servos et al., 1992), the United States (Adams et al., 1992), and New Zealand (Sharples and Evans, 1996; Sharples et al., 1994) confirming that effiuent from some discharges was capable of affecting fish at concentrations lower than previously suspected. It has since become clear that some PMEs impact the reproductive health of fish, yet the factors determining which mills will cause effects are unknown (Environment Canada, 2003; Lowell et al., 2004; Sandström, 1996).