The total respiratory impedance was measured at various frequencies (4 to 52 Hz) with a pseudo-random-noise forced oscillation technique (FOT). The apparatus (Oscillaire) was connected with a spirometer forming a closed respiratory circuit in which gas concentrations were kept constant. Measurements were made in 15 healthy subjects (group 1) and in 30 asthmatic patients with bronchial hyperreactivity, subdivided into group 2 treated only with inhaled beta 2-mimetics (n = 15) and in group 3 using both beta 2-mimetics and steroids in inhalation (n = 15). No significant differences were found between the impedance data obtained with the Oscillaire alone and those obtained with the Oscillaire connected with the spirometer circuit. The impedance was measured at FRC level, and at FRC +1 L and -1 L. The relative changes of the resistance at 8 Hz were -23.2 percent (13.8) at FRC +1 L and +40.9 percent (29.3) at FRC -1 L relative to the values at resting FRC. This inverse relation between airway resistance and lung volume was similar in all groups. The average reactance decreased at FRC -1 L in all groups. However, at FRC +1 L the average reactance increased 50.6 percent in group 2 and 94.2 percent in group 3, but decreased in group 1. Concomitant changes were observed in the resonant frequency and in the frequency dependence of resistance. Because of these qualitatively different responses of the impedance data to changes in lung volume (both for the whole group and for each individual) between healthy subjects and asthmatic patients, this test might be useful for the diagnosis of bronchial hyperreactivity.