BACKGROUND Conflicting results from previous studies on the effect of bronchial challenge on ventilatory patterns in asthmatics may be due to airflow obstruction present before induction of bronchospasm onset, as well as the different degrees of bronchoconstriction induced. PURPOSE AND METHODS We examined the ventilatory response to stepwise increments in respiratory resistance (Rrs) induced by continuous methacholine inhalation in eight patients with mild stable asthma without airway obstruction and eight normal subjects. Methacholine was inhaled continuously during tidal breathing until a nearly two-fold increase in Rrs was observed. Respiratory parameters, including tidal volume (VT), respiratory frequency (f), inspiratory ventilation (VI), mean inspiratory flow (VT/TI), and duty ratio (TI/TT), were measured simultaneously by respiratory inductive plethysmograph (RIP). Arterial oxygen saturation (SaO2) was examined by pulse oximetry. The end-tidal CO2 fraction (FETCO2) was measured with a rapid-response infrared analyzer at the mouthpiece. RESULTS Rrs, SaO2, FETCO2, and respiratory parameters, measured after saline solution inhalation, were not different between the two groups. Inhaled methacholine first decreased and then increased VI in both groups. However, changes in VI occurred earlier and to a greater extent in asthmatics than in normal subjects. At 200% Rrs (percent control), %VI was greater in asthmatics than in normal subjects (p<0.005) because of significant differences in %VT and %VT/TI between the two groups. CONCLUSION For a given degree of bronchoconstriction, the ventilatory response was more rapid and greater in patients with mild stable asthma without airway obstruction than in normal subjects. The different response to bronchial challenge between the two groups may be due to different increases in drive due to irritant receptor stimulation.