The increased minute ventilation (VE) associated with exercise produces similar degrees of airway cooling in normal and asthmatic subjects, but only those with asthma develop postexertional bronchoconstriction in response to this stimulus. We have found that when normal subjects breathing subfreezing air perform isocapnic hyperventilation to levels exceeding those associated with even exhausting exercise, 1-s forced expiratory volumes and maximum midexpiratory flow rates fall significantly. When tests more sensitive in detecting bronchoconstriction are employed, changes are seen at lower levels of hyperventilation that simulate the VE associated with moderately heavy work loads. We conclude that normal subjects respond to airway cooling, but are much less sensitive than those with asthma.