We examined the effects of chest wall strapping (CWS) on the response to inhaled methacholine (MCh) and the effects of deep inspiration (DI). Eight subjects were studied, one day with MCh inhaled without CWS (CTRL), one day with MCh inhaled during CWS (CWS_on/on), and one day with MCh inhaled during temporary removal of CWS (CWS_off/on). On the CWSon/on day, MCh caused greater increases in pulmonary resistance, upstream resistance, dynamic elastance, residual volume, and greater decreases in maximal expiratory flow than on the CTRL day. On the CWS_off/on day, the changes in these parameters with MCh were not different from the CTRL day. Six of the subjects were again studied using the same protocol on CTRL and CWS_on/on days, except that on a third day MCh was given after applying the CWS but the measurements before and after the inhalation were made without CWS (CWS_on/off). The latter sequence was associated with more severe airflow obstruction than during CTRL, but less than with CWS_on/on. The bronchodilator effects of a DI were blunted when CWS was applied during measurements (CWS_on/on and CWS_off/on) but not after it was removed (CWS_on/off). We conclude that CWS is capable of increasing airway responsiveness only when it is applied during the inhalation of the constrictor agent. We speculate that breathing at low lung volumes induced by CWS enhances airway narrowing because the airway smooth muscle is adapted at a length at which the contractile apparatus is able to generate a force greater than normal.