The metabolism of pure cis- and trans- chlordane was studied in vitro. Microsomal preparations from the livers of male rats induced with cis- or trans-chlordane in feed for ten days were used to metabolize the pure compound corresponding to the inducer. Subsequent extraction, column fractionation and combined gas chromatography-mass spectroscopy resulted in the characterization of four compounds not previously reported from an in vitro system. In addition to the substrate, trans-chlordane extracts contained species with the following molecular weights and empirical formulae: m/e 370, c 10 H 5 Cl 7 , heptachlor; m/e 352, c 10 H 6 0C1 6, a hydroxylated chlordene; and m/e 422, c 10 H 6 0C1 8 , a hydroxylated chlordane. Dichlo rochlo rdene, oxychlordane and 1- chloro- 2 -hydroxychlordene chlorohydrin were also present, With the exception of the hydroxychlordane and heptachlor, cis - chlordane extracts contained all of the metabolites found in the trans-incubates. Additionally, a fully saturated compound m/e 372, c 10 H 7 c1 7, a dihydroheptachlor, was present. The 1, 2-trans-dihydrodiol of heptachlor found in previous in vitro incubates of cis-chlordane was not present in this extract. This information has been incorporated into a proposed route for the biotransformation of the chlordanes that offers an explanation for the observed differences in the metabolism of cis - and trans-chlordane . The pathway is based on the reductive dechlorination of the chlordanes through dihydroheptachlor to dihydrochlordene. Parallel pathways of hydroxylation, desaturation and epoxide formation arise at each of these species and at chlordane itself. The trans-isomer is predominantly desaturated or hydroxylated while the cis-isomer mainly undergoes dehaloge nation.