The toxicity and metabolism of the pyrethroids cis- and trans-cypermethrin in rainbow trout, Salmo gairdneri.

1. The toxicity of cis- and trans-cypermethrin to rainbow trout was investigated and the concentrations of the two isomers in brain associated with toxic signs (excitability and loss of equilibrium) were determined. cis-Cypermethrin and trans-cypermethrin were equally toxic and showed similar brain levels associated with toxic signs (cis:0.25 micrograms/g, mean (range 0.07-0.53); trans:0.17 micrograms/g (0.07-0.31]. 2. Orally administered cypermethrin was less toxic than predicted, probably due to poor intestinal uptake. Toxicity was due to absorption via the gills of unchanged pyrethroid excreted from the intestine into the water. 3. The metabolism of the radiolabelled insecticides, [14C-cyclopropyl]- and [14C-benzyl]-cis- and trans-cypermethrin has been investigated in vivo and in vitro. 4. The principal route of elimination in vivo was the bile, with 20-28% dose excreted as biliary metabolites in 24 h. No difference in the rates of elimination of the cis and trans isomers was observed. 5. cis-Cypermethrin was metabolized primarily to the glucuronide of 4'-hydroxy-cypermethrin (80% total bile radioactivity), together with dichlorovinyldimethylcyclopropanecarboxylic acid and its glucuronide, 3-(4-hydroxyphenoxy)benzoic acid (4'-hydroxy-3BPA) and its ester and ether glucuronides, 3-phenoxybenzoyl glucuronide and 4'-hydroxy-3BPA sulphate were detected. trans-Cypermethrin was metabolized to the same products, but with only 36% as 4'-hydroxy-cypermethrin glucuronide.