Measurements of airway resistance (Raw) by body plethysmography during unconditioned air breathing are implicitly based on the assumption that the warming and humidification of air in the airways are instantaneous. Simulation with a simple model suggests that Raw may be frequency dependent and substantially underestimated at most breathing frequencies if the time constant of gas conditioning (theta) is between 0.01 and 0.3 s. We measured the frequency dependence of the real (Re) and imaginary parts of the relationship between the plethysmographic signal and airway flow from 0.5 to 3 Hz in six healthy subjects in several situations. During breathing of unconditioned air through a heated pneumotachograph, Re increased by 47 +/- 70% between 2 and 3 Hz; the data were consistent with a theta of 0.087 +/- 0.023 s. Additional dead spaces moderately increased theta: 0.105 +/- 0.031 and 0.120 +/- 0.027 s with 50-cm-long polyvinyl chloride and copper tubes, respectively. During breathing of saturated air conditioned at 34, 36, 38, and 40 degrees C, Re exhibited a much smaller positive frequency dependence, most of which was probably due to a flow dependence of Raw. We conclude that unless the inspired gas is conditioned, plethysmographic Raw is likely to be substantially underestimated, particularly when measured during spontaneous breathing.