[3H]Quinuclidinyl benzilate binding to slide-mounted frozen sections of ferret lung was of high affinity (KD 63 +/- 14 pM, mean +/- S.E., n = 4), and characteristic of interaction with muscarinic cholinergic receptors. Light microscopic autoradiography showed muscarinic receptors to be localized predominantly to smooth muscle of trachea and intrapulmonary cartilaginous airways, and to submucosal glands. There was much less labelling of bronchiolar smooth muscle, airway epithelium and vascular smooth muscle and no labelling of alveoli. This distribution of receptors parallels that of cholinergic innervation.