Exercise has been shown to result in bronchoconstriction, the extent of which is related to respiratory heat loss. Some normal subjects report coughing after exercise, and to determine whether this might also be related to respiratory heat loss, we monitored cough frequency after hyperpnea with air of varying temperature and water content in seven such subjects. Hyperpnea with fully saturated air at 37 degrees failed to provoke coughing and was not associated with heat loss or water loss. Hyperpnea with dry air at 37 degrees C resulted in more water loss (P less than 0.0005), less heat loss (P less than 0.01), and more coughing (P less than 0.001) than hyperpnea with subfreezing air. Hyperpnea with ambient air was associated with a similar cough frequency and water loss but with less heat loss (P less than 0.001) than hyperpnea with subfreezing air. There was a direct relation between cough frequency and respiratory water loss but no consistent relation between cough frequency and respiratory heat loss. This study indicates that coughing after hyperpnea with poorly conditioned air is related to the overall rate of respiratory water loss. Since strenuous exercise is associated with marked hyperpnea, coughing after exercise may also result from respiratory water loss.