Abstract.Rainbow trout were fed diets contaminated with either PCB (Aroclor 1254, 500 μg/g dry food) or Mirex (500μg/gdry food). There was no apparent effect of PCB on thyroid histology or on serum thyroid hormone levels. Mire effected significant reductions in serum T4(P<0.05) and T3levels (P<0.01) without apparent change om the histological appearance of the thyroi. Liver weights and total liver lipid content were significantly (P<0.001) greater in PCB fed fish compared with controls as was the carcass lipid content (P<0.01). Mirex had no apparent effect on liver size or liver lipid content but there was a decline in carcass lipid content (P<0.02) which persisted 25 days after returning Mirex‐fed fish to the control diet (P<0.