Larvae of the common green lacewing (GLW), Chrysopa carnea Stephens, have been reported tolerant to synthetic pyrethroid insecticides including permethrin (C/T) (3-phenoxybenzyl-3-(2,2-dichlorovinyl)-2,2-dimethyl-cyclopropanecarboxylate), the trans isomer being more toxic than cis. An investigation was performed to determine the possible role of metabolism in this tolerance. Following topical application, GLW metabolized 80% of cis and 71% of trans within two hrs. About 95% of both cis and trans were metabolized by 50 hrs. Metabolism of C/T in vitro was compared to a susceptible insect, the tobacco budworm (TBW), Heliothis virescens (F.). GLW degraded cis 1.7-fold faster than trans, while TBW metabolized trans at a slightly higher rate than cis. When esterases and oxidases were active together or alone, cis and trans were metabolized faster by GLW than TBW. Metabolism of C/T by GLW was primarily oxidative with hydrolysis as a secondary mechanism. Trans is more toxic to GLW apparently because of this isomer's lower rate of detoxication. Several metabolites of C/T, cis-, and trans-permethrin were identified in studies with GLW in vivo and in vitro. It appeared that cis was metabolized more intensively than trans in vivo. The roles of esterases and oxidases in metabolizing C/T, cis-, and trans-isomers were studied in vitro and the following number of metabolites were identified: with C/T--five with esterases plus oxidases, six with esterases, and seven with oxidases alone. With cis--six metabolites were produced when esterases plus oxidases were active, five with esterases, and four with oxidases alone. With trans--esterases plus oxidases produced four metabolites, three with esterases alone, and seven with the oxidases alone. A few unknowns were exhibited in each case. Hydroxylation at the 2'-position of the phenoxybenzyl group seems to be important for GLW tolerance to C/T. Hydroxylation could be the first step in detoxifying C/T and its isomers. The toxicity of trans to GLW could be explained by the limited routes by which esterases acting alone can degrade this isomer; only three metabolites were produced with esterases while seven were produced with oxidases.