This study was designed to examine the effects of diphenhydramine and papaverine on the toxic manifestations of cholinesterase inhibitors. It was found that papaverine increase acetylcholinesterase activity in cerebral cortex of mice. Papaverine pretreatment tended to increase acetylcholinesterase activity against the actions of neostigmine and physostigmine. When diphenhydramine (20~30 mg/kg, s.c.) was treated 20 min before the administration of cholinesterase inhibitor, it significantly extended the onset latency in the signs of toxicosis which were characteristically produced by physostigmine (0.25~1.5 mg/kg, s.c.) or neostigmine (0.125~0.5 mg/kg, s.c.), and it also prevented lethality in all of the animals.