m2 mRNA in airway smooth muscle; m3 mRNA in airway epithelium, airway smooth muscle, and submucosal glands. No detection of m4 and m5 mRNAs was observed in any cellular structures. The presence of various muscarinic receptor subtype mRNAs was confirmed by Northern blot analysis. Only human lung mRNA hybridized to the m1 probe giving a single 3.2 kb transcript. mRNA from the human cultured airway smooth muscle cells gave m2 and m3 hybridization bands of about 6.0 kb and 4.5 kb, respectively, while mRNA from the cultured airway epithelial cells gave only m3 hybridization band of 4.5 kb. With the exception of the airway epithelium, there was a good correlation between the distribution of mRNAs by in situ hybridization and the distribution of receptor subtypes by autoradiographic mapping. These results may have an important clinical implication, and also give rise to further investigation of gene regulation of pulmonary muscarinic receptor subtypes in health and disease.