We studied the relationship between force generation and airway narrowing in the major diameter bronchi (Generations 1 to 5) in 14 mongrel dogs using tantalum bronchography in vivo. A characteristic tension-radius curve was described for each airway having two phases (different elastances). Substantial heterogeneity was demonstrated in the passive characteristics of these airways; compliance increased progressively with advancing airway generation. Changes in airway caliber elicited with intravenous infusion of methacholine (MCh) were used to calculate active force translated to airway wall tensions, and radius-tension (approximately equal to length-tension) curves were calculated for each airway after correcting for wall thickness. The resting smooth muscle length at which maximal active tension (ATmax) was elicited for each generation (expressed as percent of the initial radius; %Ri) was similar for all generations (147 +/- 7.32% for Generation 2 to 160 +/- 8.82 for Generation 5). However, ATmax decreased with advancing airway generation from 6.58 +/- 1.51 dyne/cm (Generation 2, extraparenchymal) to 1.60 +/- 0.13 dyne/cm (Generation 5, intraparenchymal; p less than 0.001). Relative change in airway diameter nonetheless was greater in smaller airways (28.2 +/- 5.1% in Generation 5) than in larger airways (15.8 +/- 3.5% in Generation 2; p less than 0.001). These data indicate that increased narrowing of more distal airways in the dog results predominantly from mechanical advantage related to smaller Ri and/or greater compressibility rather than to greater force translation from airway smooth muscle contraction, as suggested by prior investigations.