A novel acetylcholine radioanalog, N-methyl-3-(l-hydroxy-5-[123I]iodopent-4-enyl)-4-acetoxypiperidine, was prepared by radioiodination of the corresponding tributylstannyl precursor that was synthesized in eight steps from 4-piperidone. The tracer has three asymmetric carbons giving eight opical isomers. Two optical isomers were isolated in the precursor synthesis by diastereomeric and enantiomeric separation. In the incubation experiments using rat cerebral homogenate, one optical isomer was hydrolyzed by acetylcholinesterase with high reactivity and selectivity. The tracer is a candidate for mapping cerebral regional acetylcholinesterase activity by single photon emission computed tomography