Cholinesterase is an enzyme associated with neural tissue. It has been demonstrated chemically and histochemically in sensory and motor tissue of the central nervous system, the myoneural junction, and the sensory end-organs.1 Specific cholinesterase, which hydrolyzes acetylcholine and acetylbetamethylcholine (methacholine), is found in autonomic nerve structures in the skin. Nonspecific (pseudo) cholinesterase, which hydrolyzes long-chain aliphatic acid esters of choline and benzoylcholine, is found in the cutaneous sensory end-organs. A group of inhibiting substances may be used to distinguish further the specific and nonspecific reactions. These may block either or both reactions, as described by Pearse.2 It has been our practice to use those inhibitors that seem best suited to the tissue being studied. Our recent experience has been with NU 683* and 62C47,† and these have proved satisfactory. It is possible to define with reasonable accuracy the type of cholinesterase activity present in a given tissue by