It has been reported that asthma provoked by breathing subfreezing air during exercise is enhanced when air at BTPS is inhaled in the recovery period (1). It was concluded that the rate of airway rewarming is an important event in asthma provoked by exercise. It is also possible, however, that the enhanced response was due to hypo-osmolarity caused by condensation of water from inspired air at BTPS on the cooled mucosa. We examined, in a group of boys with asthma, the response to rapid rewarming of the airways after exercise, with and without the potential for condensation. On two test days, two exercise tests were performed 4 h apart on a cycle ergometer. On Day 1 (n = 17), the inspired air during exercise was -5 degrees C, dry. During recovery, the air was either -5 degrees C, dry or 50 degrees C, 23 mg H2O/L. On Day 2 (n = 11), the inspired air during exercise was -15 degrees C, dry, and during recovery was either -15 degrees C, dry or at BTPS. We did not find enhancement of the response with either condition designed to cause rapid airway rewarming. On Day 1 the mean (+/- 1 SD) percent fall in FEV1 was 23 +/- 22 (-5 degrees C, dry) and 24 +/- 21 (50 degrees C, 23 mg H2O/L) (r = 0.92), and on Day 2 it was 19 +/- 17 (-15 degrees C, dry) and 18 +/- 17 (BTPS) (r = 0.96).(ABSTRACT TRUNCATED AT 250 WORDS)