Acetylcholinesterase (AChE) activity, demonstrated histochemically, defines an area of cortex on the middle ectosylvian gyrus that appears to correspond to the cytoarchitectonically defined area 41 and the physiologically defined primary auditory area (AI). In this area there are high levels of AChE in layers III, IV and VI while in the surrounding areas there are comparatively low levels of enzyme in these layers. The monoclonal antibody CAT 301, which was raised against a cell surface proteoglycan, also defines this area. There are high levels of CAT 301 immunoreactivity in cell bodies and the neuropil of layer III and an absence of very large immunoreactive neurons in layer V. Furthermore there are higher levels of the calcium binding protein, parvalbumin and the metabolic enzyme, cytochrome oxidase, in layers III and IV of AI, than in most of the surrounding cortex. By contrast the distribution of the calcium binding protein, calbindin and the distribution of myelinated fibers are similar in area 41 and the surrounding areas.