The cholinesterase activity of brains and muscles from animals which have been treated with the enzyme inhibitor sarin can be estimated by a method which removes unreacted sarin present in the tissues. The method has been used to investigate the levels of cholinesterase activity in the brain, diaphragm, and gastrocnemius muscles of rats and guinea pigs at different times after various doses of sarin. Brain cholinesterase activity was found to be the earliest and most easily inhibited, followed by that of the diaphragm and gastrocnemius muscles. Spastic paralysis of the hind limbs of rats could be induced by sarin without any appreciable inhibition of the cholinesterase activity of their gastrocnemius muscles. The brains of rats and guinea pigs showed an appreciable natural reactivation of the cholinesterase activities after sublethal doses of sarin. This effect could not be duplicated in vitro.