Ten initially asymptomatic asthmatic subjects underwent methacholine-induced attacks of asthma on a number of occasions, to the point when tightness in the chest was just sensed (threshold point). As baseline lung volumes increased and the caliber of the airways decreased, the relative changes needed to attain a threshold point generally became smaller. Although pulmonary function can deteriorate chronically without increasing dyspnea, it is possible that a mechanism exists at the level of consciousness to protect a subject's breathing at times when pulmonary function is acutely impaired.