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Roach (Rutilus rutilus) were exposed, along with their appropriate controls, to a simulated bleached kraft mill effluent (KME-Sa + CP) first for 38 days at a concentration of 0.035 X LC50 and then for 14 days at 0.07 X LC50. During the experiment, their tolerances to KME-Sa + CP were tested five times by measuring the 48-hr LC50 values. In addition, the growth of roach was monitored. At the end of the exposure, accumulation of [14C]pentachlorophenol in various parts of the fish (total PCP in water 15.6 micrograms/liter) was measured. When the fish were preexposed to KME-Sa + CP, the acute tolerance of this mixture in roach increased by 30-39%, but the response was abolished in 31 days. Fish growth remained unchanged during the experiment. Measurement of PCP accumulation revealed no difference in the absorption rate, but under steady-state conditions the degree of bioconcentration was 16% lower (P less than 0.02) in preexposed roach than in their unexposed controls. This difference was entirely accounted for in the head and visceral parts of the fish. Even when no final changes were noted in tolerance and growth rate of the fish, the authors suggest that the significantly lowered body burden implied acclimatizatory compensation under subchronic exposure of this xenobiotic.