Airways of 2-5 mm in diameter were dissected from fresh dog lungs and mounted in a tissue bath so that they could be inflated and deflated with air. Length was held constant, and pressure and volume were monitored on an X-Y recorder. Pressure-volume curves were obtained for each airway at several difference degrees of constriction, by varying the concentration of acetylcholine (ACh) in the bath. Properties of the tissue were analyzed cy cycling airways at different speeds and by inflating them to various pressures. Circumferential length-tension curves were calculated from the inflation limbs of pressure-volume curves obtained by slow cycling (90 s for a complete cycle between -10 and 40 cmH2O). Administration of a series of increasing concentrations of ACh resulted in length-tension curves that were displaced to shorter lengths but not greatly altered in slope. These results suggest that slow inflation of the bronchus stretched only passive elements in series with a contractile element which shortened to the length determined by the concentration of ACh present.