The distribution of messenger RNA coding for acetylcholinesterase was studied in human post mortem brain and rhesus monkey by in situ hybridization histochemistry and compared to the distribution of acetylcholinesterase activity. Acetylcholinesterase messenger RNA had--similar to acetylcholinesterase enzymatic activity--a widespread distribution in human bain. Acetylcholinesterase messenger RNA positive cells corresponded to perikarya rich in acetylcholinesterase activity in most but not all regions. Examples for mismatches included the inferior olive and human cerebellar cortex. The presence of hybridization signals in cerebral cortex and an enrichment in layer III and V of most isocortical areas confirmed that perikaryal acetylcholinesterase in cerebral cortex is of postsynaptic origin and not derived from cholinergic projections. In striatum the expression of high levels of acetylcholinesterase messenger RNA was restricted to a small population of large striatal neurons. In addition, low levels of expression were found in most medium sized striatal neurons. Cholinergic neurons tended to express high levels of acetylcholinesterase messenger RNA whereas in cholinoceptive neurons the levels were moderate to low. However, some noncholinergic neurons like dopaminergic cells in substantia nigra, noradrenergic cells in locus coeruleus, serotoninergic cells in raphé dorsalis, GABAergic cells in thalamic reticular nucleus, granular cells in cerebellar cortex and pontine relay neurons expressed levels comparable to cholinergic neurons in basal forebrain. It is suggested that neurons expressing high levels of acetylcholinesterase messenger RNA may synthesize acetylcholinesterase for axonal transport whereas neurons with an expression of acetylcholinesterase confined to somatodendritic regions tend to contain lower levels of acetylcholinesterase messenger RNA.