Aqueous extracts were prepared from leaves of Ricinus Communis, L. by centrifuging homogenates at 18000 g for 20 min. Extracts prepared from yellow or brown leaves inhibited the oxidation of succinate by mitochondria prepared from Ricinus endosperm. Extracts became less inhibitory when dialysed or treated with bovine serum albumen. The inhibitory properties of the extracts are attributed to their fatty acid content.