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Physiological and biochemical biomarker responses were studied in juvenile whitefish (Coregonus lavaretus L. s.l.) exposed experimentallyto effluent from the forest industry. The large study area (609 km2), Southern Lake Saimaa, in Southeast Finland, receives 330,000 m3 d-1 of biologically and 55,000 m3 d-1 of chemically treated effluents, discharged fromtwo integrated elementary chlorine free (ECF) bleached kraft pulp and paper mills, from one ECF pulp mill, and from one mill producing unbleached pulp and cardboard. The assessment of exposure to effluent discharged from the mills was based on lake water chlorophenolics (CPs) and resin acids (RAs) measured in samples collected from the 22 experimental sites along the area. Despite the low levels of effluent constituents in the lake, they were still accumulated in detectable levels in fish bile, indicating an exposure to the bioactive compounds of effluents. In comparison to the reference area, a two- to four-foldincrease in ethoxyresorufin O-deethylase (EROD) activity was observed in whitefish exposed in the vicinity (1--6 km) of all the mills. However, cytochrome P450 1A1 (CYP1A1) gene expression was increased in only one of the receiving areas, indicating higher sensitivity of theEROD activity in the present study. There were no statistically significant correlations between EROD activity and the ambient water concentrations of the CPs, the RAs, or effluent dilution expressed by water sodium concentration. Neither bile chlorophenolics nor bile resin acids showed a significant correlation with EROD. No significant changes in circulating reproductive steroids, 17beta-estradiol and testosterone, in juvenile whitefish were observed. The vitellogenin gene was expressed in the vicinity of the pulp mill discharging the most wood-derived compounds, ie. resin acids and wood-sterols, including beta-sitosterol. No differences were observed in plasma immunoglobulin M,gluc [ABSTRACT FROM AUTHOR]