PESTAB. The effects of various insecticides on energy production by bluegill, Lepomis macrochirus, liver mitochondria have been investigated. The studies were made in the presence of succinate and alpha-ketoglutarate as substrates. The organochlorine insecticide dieldrin and the carbamate insecticide carbofuran did not alter the oxygen or phosphate uptake in the presence of either substrate. The remaining organochlorine insecticides, aldrin, chlordane, DDE, endrin, heptachlor, kepone (chlordecone), lindane, methoxychlor, and toxaphene, at a level of 0.1 mg/ml, altered oxygen uptake to various extents ranging from 25% inhibition for aldrin to 87% inhibition in the presence of heptachlor. The organochlorine insecticides did not alter phosphate uptake in the presence of succinate. All the organophosphate insecticides, dasanit (fensulfothion), diazinon, malathion, parathion, and thimet (phorate), inhibited oxygen uptake. The inhibition of oxygen uptake by bluegill liver mitochondria was more severe in the presence of organophosphate insecticides than in the presence of organochlorine insecticides. Phosphate uptake in the presence of succinate was not altered in the presence of organophosphate insecticides. Sevin (carbaryl) inhibited oxygen uptake 35% and was the only carbamate insecticide to alter either oxygen or phosphate uptake in the presence of either substrate. Chlordane and hepatchlor inhibited oxygen uptake by bluegill liver mitochondria 39 and 29%, respectively, in the presence of alpha-ketoglutarate. No effect on phosphate uptake was noted. Lindane did not change oxygen uptake but decreasee the uptake of phosphate. Methoxychlor increased oxygen uptake. The other organochlorine insecticides did not alter either oxygen or phosphate uptake by the bluegill liver mitochondria. Diazinon, malathion, and thimet inhibited oxygen uptake for 60% for thimet to 88% for diazinon. Dasanit and parathion did not alter either oxygen or phosphate uptake. Carbofuran and sevin did not change oxygen or phosphate uptake in the presence of alpha-ketoglutarate.