Maleic anhydride is recognized as a very active skin irritant. Single doses were administered by mouth and by skin absorption to determine if its internal toxicity is correspondingly high. The sample was received 1-3-40 from South Charleston on plant order S-38902. It was of the usual commercial grade. The LD(sub 50) for rats by mouth was found to be 1.1 gm./kg., and for guinea pigs 0.7 gm./kg. Even 5 gm./kg. in the form of a 10% solution did not kill guinea pigs when kept four days on the skin. Death was rapid. Bile was orange or red, kidney was pale, liver was congested, stomach was congested and injected for doses by mouth. Death was apparently due largely to shock and digestive tract irritation, accompanied by injury to liver and kidney. The 10% aqueous solution left on guinea pig skin for four days produced slight local necrosis and a papular eruption, without any internal changes. Apparently little penetrated the skin. Some applications of melted anhydrous maleic anhydride were made. All animals died, even at a dosage of 1 gm./kg. Death was apparently due to shock and infection following the very severe local necrosis produced. In dilute solutions by mouth maleic anhydride is moderately but not extremely toxic. On the skin a 10% solution is slightly necrotic, but does not penetrate appreciably into the body.