ABSTRACT.Muscarinic cholinergic receptors were identified and partially characterized in crude membrane fractions of rat lung and trachea from day 17 of gestation to adulthood using (-)-[3H]quinuclidinyl benzilate (QNB). (–)-[3H]QNB binding to rat lung membrane was characteristic of muscarinic cholinergic receptor sites. Binding capacity of muscarinic receptors sites was relatively low in rat lung compared to that in other tissues. The number of (–)-[3H]QNB-binding sites (binding capacity) decreased progressively and significantly from 79 ± 8 fmol mg−1protein on days 17–18 of gestation to 21 ± 3 fmol mg−1mean ± SEM on days 21–22 of gestation, p < 0.01. Binding capacity did not vary thereafter from birth to adulthood. Affinity of (-)-[3H]QNB binding for lung membranes did not change with age (KDapproximately 70 pM). (–)-[3H] QNB-binding sites were significantly higher in membrane preparations of trachea or tracheal-bronchial tissue than in lung parenchyma from both the adult and newborn rats. (-)-[3H]QNB binding was undetectable in crude membrane preparations of cultured purified type II epithelial cells isolated from the adult rat lung.