We studied the relationship of airway morphometry, the content of myosin heavy-chain and isoform stoichiometry, and the distribution of bronchoconstrictor responses in the airways of maturing swine. Lungs were excised in 2-wk-old farm swine (2ws; n = 13) and 10-wk-old swine (10ws; n = 13), and tracheal smooth muscle strips and bronchial rings from generations 2-5 were fixed for in vitro isometric measurement of force generation. Split samples were placed in formaldehyde solution or glutaraldehyde for light- or electron-microscopic morphometry or frozen for analysis of tissue myosin content. The rank order of force generation elicited by both receptor- and nonreceptor-dependent mechanisms for both 2ws and 10ws was generation 4 greater than 3 greater than or equal to 2. For all matched airway generations, contractile force was 25-100% greater in 2ws than 10ws. Differences in force generation were not related to morphometric differences in smooth muscle mass content among airways. The relative cross-sectional area of smooth muscle derived by computerized morphometry was 5.5-7% for each airway generation and did not change with age. Electron-microscopic morphometry demonstrated comparable myocyte content within muscle bundles for all airways in both age groups. In generation 4 airways, myocyte size in 2ws (27.3 +/- 0.8 nuclei/2,500 microns2) hypertrophied approximately 15% in 10ws (20.4 +/- 0.6 nuclei/2,500 microns2; P less than 0.01). Tissue content of myosin measured by computerized laser densitometry of gel electrophoresis of homogenates was greater in trachea from 2ws than 10ws (135 +/- 10 vs. 90 +/- 4 micrograms/g tissue; P less than 0.01); homology of 200- and 205-kDa isoforms was confirmed by Western blot against polyclonal myosin antibody and Cleveland digest analysis of each band. Differences in contractile forces between generations in 2ws and 10ws were not correlated to functional myosin isoform content. We demonstrate a maturational downregulation of contractile forces in maturing swine. This response is independent of smooth muscle receptor distribution and is not related to morphological changes in airways muscle mass, cellularity, changes in content of nonmyocyte tissues, or tissue content of functional myosin isoform.