The cellular distribution of muscarinic acetylcholine receptor protein in the frontal cortex of Alzheimer (AD) patients, age-matched and middle-aged controls was assessed quantitatively by means of immunohistochemistry using the monoclonal antibody M35. As shown previously in biopsy cortices, mainly layer II/III and V pyramidal neurons were immunolabeled. Neither distribution nor numbers of labeled cells displayed significant differences between the groups investigated. This is in accordance with the results of ligand binding studies that mostly failed to reveal different binding characteristics in AD compared to controls. Muscarinic and nicotinic receptor proteins have been shown to be colocalized in many cholinoceptive pyramidal neurons. Since nicotinic receptors--in contrast to muscarinic receptor proteins--are severely reduced in AD, this indicates a selective impairment of nicotinic receptor expression and not a significant death of cholinoceptive neurons per se.