The metabolism of (14)C labelled trans cypermethrin by a pyrethroid resistant strain (PEG87) of Heliothis virescens was studied in vivo to characterise the enzymes responsible for resistance to pyrethroids. When 3rd instar larvae were dosed with piperonyl butoxide (PBO), there was strong inhibition of oxidative metabolism of trans cypermethrin to the metabolites 20H/40H trans cypermethrin, and a consequently slow rate of excretion of conjugates of labelled metabolites in excreta. In vitro studies on 4th instar larvae showed rapid oxidative metabolism to 20H/40H trans cypermethrin by microsomes, which was NADP-dependent. Relatively slow esterase hydrolysis occurred in 11,000g supernatants, microsomes, and 105,000g supernatants. These results provide strong evidence that a microsomal monooxygenase system with cytochrome P-450 as its catalytic centre, is a major factor in the resistance of PEG87 insects to pyrethroids. Further work on this system will give insight into potential problems of cross-resistance and identify synergists that may counteract resistance in the case of pyrethroid insecticides. Purification of the enzyme can lead to the development of a diagnostic kit for the identification of this type of resistance in the field. The possibility that an exterase also contributes to resistance is currently under investigation.