Isocapnic hyperventilation with subfreezing air was performed by 15 healthy non-smokers, 10 asymptomatic smokers and 9 asthmatics. All subjects had normal ventilatory function and airway resistance (Raw) before challenge. The hyperventilation was performed twice. In one session, total respiratory resistance (Rrs) and reactance (Xrs) were measured at various frequencies, using a forced oscillation technique; in another session, vital capacity (VC), forced expiratory volume in 1 s (FEV1), maximal expiratory flow rates (FEF) and Raw were determined. In non-smokers, no changes in FEV1, FEF nor Raw were observed, whereas Rrs increased significantly (+ 20% of the prechallenge value), without change in resonant frequency nor in the Rrs-frequency relationship. This suggests a constrictory effect on central airways (possibly a narrowing of the glottis) only. In smokers, Rrs showed a similar, though longer lasting, increase than in non-smokers. Besides, a significant change of the Rrs-frequency relationship and a reduction in FEF at 50% of VC was found, suggesting an involvement of peripheral airways also. In asthmatics, bronchial reactivity was more pronounced, resulting in significant changes in all parameters: Rrs increased by about 100% of the prechallenge value, and became highly frequency dependent; Xrs decreased markedly, resulting in an increase in resonant frequency of the respiratory system. Similarly, VC, FEV1 and FEF decreased. These alterations are compatible with a more generalized constriction of the peripheral as well as central airways.