In 10 asymptomatic young smokers and 15 non-smokers, total respiratory impedance was determined by means of the forced oscillation technique in a frequency range between 4 and 52 Hz under basal conditions, and after isocapnic hyperventilation with cold air. In non-smokers, cold-air inhalation caused a statistically significant increase in respiratory resistance between 8 and 28 Hz, and post-challenge reactance values decreased at almost all frequencies. In smokers, respiratory resistance increased at almost all frequencies, but most markedly at lower frequencies in the range from 8 to 28 Hz; respiratory resistance decreased with increasing frequency. Respiratory reactance decreased significantly between 8 and 36 Hz. Comparing the smokers and non-smokers, respiratory resistance and reactance were not significantly different under basal conditions. However, after cold-air challenge, respiratory resistance was significantly higher between 8 and 16 Hz, and respiratory reactance significantly lower between 8 and 40 Hz. It is concluded that after cold-air challenge, a broncho-constricting effect extending largely into the small peripheral airways can be demonstrated by impedance measurement in a group of asymptomatic young smokers which is not observed in normal subjects. Therefore, this method is very useful for the early detection of mechanical abnormalities induced by smoke inhalation.