Alterations of muscarinic acetylcholine receptors in the nasal mucosa of allergic patients in comparison with nonallergic individuals. Cholinergic nasal hyperresponsiveness in nasal allergy may be due to changes of the characteristics in muscarinic cholinergic receptors. Radioligand receptor binding and in vitro autoradiographic studies of nasal mucosa in nonallergic (NA) and allergic patients were performed to investigate this hypothesis. The heterogeneous NA group was subdivided into control individuals and patients with chronic sinusitis and vasomotor rhinitis. The 3H-(-)-Quinuclidinylbenzilate binding to muscarinic receptors in human nasal mucosa membranes was saturable and of high affinity in all groups. No significant differences could be demonstrated between the subgroups of the NA patients. In allergic patients the dissociation constants and receptor densities were significantly decreased in comparison with those of NA and with those of control individuals. No differences in agonist binding or coupling of the muscarinic receptor to the effector system via the G protein could be observed in allergic patients. In vitro autoradiographic experiments demonstrated specific 3H-(-)-Quinuclidinylbenzilate labeling of the glandular acini in NA and allergic patients. No specific labeling could be observed in the epithelium, blood vessels, or connective tissue. In conclusion, the increased sensitivity and decreased muscarinic receptor number may reflect the cholinergic-induced hypersecretion in nasal allergy but are probably too small to explain the complex allergic reaction. 