An in vivo rainbow trout (Salmo gairdneri) preparation was used to evaluate the gill uptake and toxicokinetics of [³H]fenvalerate ([R,S]-α-cyano-3-phenoxybenzyl [R,S]-2-[4-chloro-phenyl]-3-methylbutyrate), a synthetic pyrethroid insecticide. Fish were exposed to technical-grade fenvalerate (0.28 or 23 ng/L) or an emulsifiable-concentrate formulation (16 ng/L) for 36 to 48 h. No significant effects of emulsifiers or fenvalerate concentration on uptake were observed. The overall mean gill uptake efficiency was determined to be 28.6 + 4.4%. Following 8- to 48-h depuration periods, carcass and bile contained 80 to 90% and 10 to 20% of the gill-absorbed doses, respectively. Urine, feces and blood each contained less than 2% of the dose. Significant excretion and blood transport of fenvalerate equivalents were completed within 8 to 12 h after termination of exposure. Specific tissues from trout exposed to 0.28 ng/L fenvalerate were analyzed for fenvalerate equivalents. After a 48-h depuration period, bile contained the highest concentration of fenvalerate equivalents (7,000 pg/g), followed by fat (200 pg/g). Remaining tissues contained 15 to 45 pg/g. Analysis of biliary metabolites indicated that the glucuronide of 4′-HO-fenvalerate was the only significant degradation product. Results from the present study suggest that efficient gill uptake does not explain the extreme sensitivity of fish to fenvalerate. Rather, a low rate of biotransformation and excretion may play a significant role in the susceptibility of rainbow trout to the synthetic pyrethroid insecticides.