Nasal mucosal blood flow and nasal patency were studied in seven healthy subjects in order to evaluate normal physiological reactions to cold and heat. Blood flow was measured by means of the 133Xe washout method, and nasal patency was determined by measuring maximal nasal expiratory air flow. In comparison with the results at room temperature (23 degrees C), there was a decreased blood flow and a decreased nasal patency after 20 minutes of exposure to cold (6 degrees C). After exposure to heat (40 degrees C), an unaltered blood flow and an increased nasal patency were registered. The results suggest that nasal patency is important for the conditioning of air and support the hypothesis that changes in nasal mucosal blood flow is engaged in body temperature regulation.