To compare the importance of water loss with heat loss in the mechanism of exercise-induced asthma (EIA), we conditioned the air inspired during exercise to the same water content (Wi) while changing its temperature (Ti), and vice versa. We calculated separately the amount of heat and water required to bring the inspired air to alveolar conditions. Ten asthmatics ran for 8 min on 4 occasions in an environmental chamber. The air was conditioned to a Ti of 9 to 10 degrees C or 35 degrees C, with a Wi of 9 to 10 mg H2OL-1. The airway response to exercise was not significantly different when Ti varied by 25 degrees C, but the amount of water required to saturate the inspired air remained the same. However, the amount of heat required was significantly greater with cooler air (p less than 0.005). We conclude that it is the osmotic and not the cooling effects induced by the vaporization of water that is the more important factor determining EIA.