We report a new device for examining the bronchial hyperresponsiveness by directly writing the dose-response curve of respiratory resistance (Rrs) during the continuous inhalation of the methacholine in stepwise incremental concentrations. Respiratory resistance was measured by the forced oscillation method. We found that the Rrs began to increase at a certain threshold concentration of methacholine (bronchial sensitivity) and that it has a curvilinear slope (bronchial reactivity). Subsequent inhalation of the bronchodilator drug returned the Rrs to the control level. Thus, we were able to examine bronchohyperresponsiveness in the patterns of the cumulative dose-response curves of methacholine. All normal subjects were nonresponders; while all of the asthmatic subjects, 63 percent (10) of the 16 patients with chronic bronchitis and 50 percent (7) of the 14 patients with acute bronchitis were responders. The dose-response curves were reproducible. Our device may be clinically applicable for examinations of bronchial hyperresponsiveness and for screening tests.