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The effects of thermomechanical pulping effluents from an integrated newsprint mill were tested on rainbow trout (Oncorhynchus mykiss). Continuous‐flow exposures under laboratory conditions were conducted at three dilutions (1:200, 1:400, and 1:1,000) for 8 weeks, followed by a 4‐week recovery period. Sublethal effects were assessed using physiological and biochemical parameters including liver histology, hematology, serum biochemistry, and hepatic enzyme assays. Exposure was verified by analyzing water column, fish bile, and fish tissues for resin‐acid concentrations. The effluent was found to be lethally toxic to rainbow trout within 3 to 4 weeks at a dilution of 1:200. Resin‐acid concentrations in bile, muscle, gill, and liver showed a clear‐cut positive dose response. However, relatively few responses and changes in the physiological parameters analyzed were found when comparing exposed and reference fishes. The small differences in physiological parameters between the reference group and an exposure group (1:400) after a 4‐week recovery period show that the observed responses were reversible. It is noteworthy that the difference between the lowest acutely toxic concentration and the concentration at which only slight physiological responses occurred was very narrow for this effluent.