Two-year-old rainbow trout (Salmo gairdneri), reared for 2 yr in water containing about 0.00025 mg/ℓ hexavalent chromium (Cr+6) (Naches trout) or between 0.002 and 0.010 mg/ℓCr+6(Hanford trout) accumulated appreciable chromium, yielding whole body residues of about 0.029 and 0.18 μg/g wet tissue, respectively. Highest concentrations were in the opercular bone, spleen, kidney, gastrointestinal tract, and gall bladder. Short-term exposure of Hanford trout to 2.5 mg/ℓCr+6caused a rapid additional increase of tissue chromium, but at 22 days whole body levels were only 0.87 μg/g. Upon return of exposed fish to water containing 0.002–0.010 mg/ℓ chromium, the metal was rapidly depleted from most tissues except kidney, liver, gill, gall bladder, and bile. Chromium accumulated in tissues of trout exposed to 2.5 mg/ℓ Cr+6was not distributed proportionally among the various subcellular fractions but concentrated in the cell cytosol, especially in the liver and gill.Mitochondrial cytochrome oxidase, NADH-cytochrome c reductase, and succinate cytochrome c reductase activities in liver, kidney, gill, and brain tissues of Naches trout, Hanford trout, and Hanford trout exposed to 2.5 mg/ℓ Cr+6were not significantly different except for kidney NADH-cytochrome c reductase which was lower in Hanford and chromium treated fish. Microsomal nitroreductase,O-demethylase and NADPH-cytochrome c reductase and the soluble glucose-6-phosphate dehydrogenase activities in liver and kidney from Hanford trout were significantly lower than those of Naches trout. Exposure of Hanford trout to 2.5 mg/ℓ Cr+6, however, did not reduce the activities of these enzymes below control levels. In vitro studies showed that trout enzymes were fairly insensitive to Cr+6inhibition. These results suggest that observed differences in enzyme activity between Naches and Hanford trout may be caused by factors other than chromium content