(HCH) is converted, on incubation with the IOOOOOg supernatant fraction (cytosol) of rat liver homogenates, to four positional isomers of S-(dichloropheny1)glutathione (DCPG). 2. Radiochemical evidence shows that primary conjugates formed are subject to rapid aromatization. 3. The GSH-conjugates produced from y-HCH (Iindane), 6-HCH, and three stereoisomers of 1,3,4,5,6-pentachlorocyclohex-1 -ene (PCCH) have been characterized by g.1.c. of their aromatic moieties. All compounds were exclusively converted to at least two positional isomers of DCPG. An isomer of HCH and its trans-dehydrochlorination product yielded DCPG of almost identical composition. 4. 8-HCH was not entirely unreactive, but the identity of the product remained uncertain. 5. DCPG-formation from HCH-d, exhibited a significant deuterium isotope effect (5.8 at 310 K for the a-configuration), while none was found for the conversion of PCCH-d5'(1.2 at 298 K for the 3,4,6/5-isomer). 6. In the absence of GSH, liver cytosol protein mediated a trans-dehydrochlorination of lindane and of a-HCH to PCCH with a catalytic factor of 15 and 25, respectively. Addition of GSH raised HCH conversion by afactor of 3 to 4 and resulted in the formation of DCPG. 7. GSH-conjugation of PCCH is shown to be enzymic. 8. It is concluded that the rate of formation of DCPG from HCH in rat liver cytosol depends on gradual monodehydrochlorination, and that the enzymic transfer of GSH onto PCCH is not preceded by a second dehydrochlorination. The transfer and elimination reactions involved in DCPG formation from PCCH are considered taking into account (a) differences between stereoisomers in reaction rate and product composition and (b) the observation that a purified soluble GSH-S-transferase (E.C. 2.5.1.18) converted 3,4,6/5PCCH-and a-HCH-to the same set of four isomeric DCPGs as did the entire cytosol fraction. 9. In corroboration of earlier evidence, transferase activity associated with liver microsomes and mitochondria also converts a-HCH and 3,4,6/5-PCCH each to four positional isomers of DCPG. 10. The result of the study is discussed with reference to earlier work in mammals and in insects and in relation to HCH-biotransformation by rats in vivo. Abbreviations: