It has been suggested that nasal breathing attenuates the airway obstruction that follows physical exertion in asthmatics. In an effort to determine the reason for this protection, we had nine asymptomatic asthmatics and five normal subjects inhale subfreezing air at equal ventilations through either their noses or mouths in a random fashion while we measured the temperature in the retrotracheal esophagus (Trt). Pulmonary mechanics recorded before and after voluntary eucapnic hyperventilation simulating moderately heavy workloads demonstrated a mean fall in forced expiratory volume in one second (as a representative variable) of 28.6% +/- 4.8% (SEM) and 7.5% +/- 1.9% from control in the oral and nasal challenges, respectively, in the asthmatic subjects (p less than 0.001). Measurement of Trt during hyperventilation showed a mean fall of 2.7 degrees +/- 0.05 degree C with nasal breathing in this group (p less than 0.0001) and a linear relationship between the degree of airway cooling and the severity of subsequent bronchoconstriction (r=0.81). The normal subject showed similar changes in temperature but did not change their lung function. These data demonstrate that nasal ventilation minimizes airway cooling in both normal and asthmatic individuals through more efficient conditioning of inspired air, and it is through this mechanism that this form of respiration protects against exercise-induced bronchospasm.