It has been shown that airway smooth muscle in vitro is able to maintain active force over a large length range by adaptation in the absence of periodic stimulations at 4 degrees C (Wang L, Paré PD, and Seow CY. J Appl Physiol 90: 734-740, 2001). In this study, we show that such adaptation also takes place at body temperature and that long-term adaptation results in irreversible functional change in the muscle that could lead to airway hyperresponsiveness. Rabbit tracheal muscle explants were passively maintained at shortened and in situ length for 3 and 7-8 days in culture media; the length-tension relationship was then examined. The length associated with maximal force generation decreased by 10.5 +/- 3.8% (SE) after 3 days and 37.7 +/- 8.5% after 7 or 8 days of passive shortening. At day 3, the left shift in the length-tension curve due to adaptation at short lengths was reversible by readapting the muscle at a longer length. The shift was, however, not completely reversible after 7 days. The results suggest that long-term adaptation of airway smooth muscle could lead to increased muscle stiffness and force-generating ability at short lengths. Under in vivo condition, this could translate into resistance to stretch-induced relaxation and excessive airway narrowing.