1. The LC50 and the uptake and elimination kinetics of 4-chlorodiphenyl ether (4-CDE), 2,4-dichlorodiphenyl ether (2,4-diCDE), 2,4,4'-trichlorodiphenyl ether (2,4,4'-triCDE) and 2,4,5,4'-tetrachlorodiphenyl ether (2,4,5,4'-tetraCDE) were examined in brook trout (Salvelinus fontinalis). The chlorinated diphenyl ethers (CDE) were administered in the water. 2. The 96 h LC50 of 4-CDE and 2,4-diCDE were 0.73 and 0.66 mg/l, respectively. The LC50 of 2,4,4'-triCDE and 2,4,5,4'-tetraCDE could not be determined since they were greater than the water solubilities of these chemicals. 3. Uptake of the CDE by trout was rapid; the uptake rate ranged between 2.4 and 48.9 micrograms/day. However, CDE uptake by trout did not reach a steady state at the conclusion of a 7 day exposure. In contrast, depuration of the CDE from trout was slow; the depuration half-lives of the CDE ranged between 3.9 and 63 days. 4. The toxicokinetics of the CDE in trout could be described by a one-compartment open model with zero-order absorption and first-order elimination. The CDE were taken up initially into the blood and liver of trout before they were redistributed to adipose tissue and muscle. 5. CDE are significantly bioaccumulated by fish from water; the model-predicted maximum body burdens of trout exposed to 100 micrograms/l CDE in water are 146, 316, 1381, and 930 micrograms/g for 4-CDE, 2,4-CDE, 2,4,4'-triCDE and 2,4,5,4'-tetraCDE, respectively.