Pretreatment of rats with gamma HCH, DDT, and DDT + gamma HCH accelerates the metabolism of gamma HCH. A single oral dose of 14C gamma HCH was administered to all animals after 2 wk of pretreatment. The treated rats excreted significantly more radioactivity and stored significantly less 14C gamma HCH than the control animals. In addition to significantly larger livers, the treated rats exhibited higher cytochrome P-450 content and greater in vitro enzyme activity than the control animals. The self-induction of gamma HCH metabolism, which has not previously been reported, could be of major importance in establishing a steady-state relationship between exposure and storage of such pesticides by mammals. Both quantitative and qualitative differences in gamma HCH metabolism are observed between the treated groups of rats. DDT pretreatment results in a 35% higher excretion of radioactivity than gamma HCH pretreatment.