The stimulation of (14C)lindane metabolism in rats by pretreatment with DDT or lindane has been previously demonstrated in this laboratory. However, it appeared that the mechanism by which lindane induced its own degradation differed from the mechanism by which DDT stimulated lindane metabolism. The present study attempted to clarify some of the differences in the metabolism of lindane resulting from pretreatment of rats with DDT and with lindane itself. Daily treatment of rats with either lindane alone or with DDT plus lindane permitted an evaluation of the comparative urinary excretion of the lindane-derived chlorophenols. Rats receiving DDT plus lindane excreted significantly more 2,4,5-trichlorophenol and 2,3,4,6- and 2,3,4,5-tetrachlorophenols by the second day of treatment than did rats receiving lindane alone. Results of the study suggest that DDT treatment stimulates the metabolism of lindane through a selective effect on certain metabolic pathways involved in the oxidative degradation of lindane.