In a study of the hepatotoxicity of heptacholor (1, 4, 5, 6, 7, 8, 8-heptachloro 3a, 4, 7, 7 a-tetrahydro-4, 7-methanoindene), a major compound of chlordane, the effect of heptachlor on the respiratory activity (oxidative phosphorylation and electron transport) of rat liver mitochondria was investigated. Heptachlor at a final concentration of 50 μM with succinate as substrate decreased the respiratory control index (RCI) due to a marked inhibition of state 3 respiration and a slight inhibition of state 4 respiration. One hundred uM heptachlor with succinate as substrate suppressed the states 3 and 4 respiration almost completely. On the other hand, heptacholor at a final concentration ranging from 50 to 100 μM with β-hydroxybutylate (β-HB) slightly decreased the RCI and decreased the RCI hardly at all with ascorbate plus N, N, N', N', -tetramethylphenylene diamine (TMPD) as substrate. Hepta-chlor at a concentration of 50 μM in the presence of succinate also decreased the ADP/O ratio of mitochondria. The mode of inhibition of succinate oxidation by heptacholor apparently is a noncompetitive inhibiton, as shown by Lineweaver-Burk plot.