Two enzymes involved in mitqchondrial electron transport in the catfish,Ictalurus melas, have been shown to be inhibited by endrin. Succinate dehydrogenase and cytochrome oxidase were both inhibited, and the extent of inhibition depended ypon endrin concentration and the specific activity ef the enzyme preparation. No appreciable effect of endrin on acetylcholinesterase or NADH-cytochrome c reductase was seen. Binding to lipoprotein components essential for mitochondrial oxidation has been proposed as a logical site for endrin action.