Ten asthmatic and 10 normal subjects were exposed to 7 air conditions varying in temperature (-2 to 49 degrees C) and relative humidity (10 or 100%). Normocapnic hyperventilation for 3 min at 40% maximal voluntary ventilation was performed for each condition. A constant-volume body plethysmograph measured functional residual capacity and specific airway conductance (SGaw). Three to five sets of functional residual capacity and SGaw measurements were taken before and 1, 5, and 10 min after each challenge. All conditions that extracted heat from the airway caused SGaw to fall in both subject groups. In normal subjects, this response was greatest immediately after challenge. In asthmatic subjects, the immediate response exceeded the normal response, and airway function worsened in the 5 to 10 min after exposure. The quantity of heat extracted correlated moderately well with change from baseline SGaw in both normal (r = 0.55 at 5 min) and asthmatic (r = 0.64) subjects. When heat was added to the airway by hyperventilation of warm, humid air, SGaw increased in normal subjects. However, marked bronchoconstriction followed the same challenge in asthmatic subjects. Changing osmolality of the respiratory mucosa rather than thermal exchange may be responsible for the bronchospasm consequent to heat delivery.