The comparative effect of 7 days pre-treatment with one of seven organochlorine pesticides on the metabolism of lindane in vivo and on the metabolism of EPN, p-nitroanisole and methyl orange in vitro was investigated. Mirex was the most potent inducer of the oxidative hydrolysis of EPN, the O-demethylation of p-nitroanisole and the azo reduction of methyl orange. Chlordane and DDT were equipotent followed by hexachlorobenzene, toxaphene, pentac and penphene. DDT stimulated significantly greater metabolism of lindane than did pre-treatment with any of the other six pesticides. In general DDT, mirex, chlordane and hexachlorobenzene were the most effective activators of lindane metabolism followed by toxaphene, pentac and penphene. Comparative differences in lindane metabolism in vivo were not adequately reflected by the determination of enzyme activities in vitro. Pre-treatment with organochlorine pesticides altered lindane metabolism by selective effects on specific metabolic pathways.