We investigated the effects of breathing air warmed and fully saturated to body temperature (AWS) before, during, and after exercise in asthmatic subjects. Airway responses to submaximal exercise on a cycloergometer were measured on four separate days in 14 asthmatic volunteers. On day 1 the subjects exercised breathing ambient air (AA). On the subsequent three days exercise was performed with the subjects breathing AWS, (1) for five minutes preceding, (2) during, and (3) for five minutes following exercise. We showed complete protection against EIB by AWS during exercise, but no protection by AWS before or after exercise. On two subsequent days we examined the effects of partially warming and humidifying the subjects' inspired air by having them wear a mask during exercise. We found that with such protection bronchospasm was significantly but not completely blunted. We conclude that the physiologic changes initiated during exercise can be prevented by breathing AWS during exercise, but are not by AWS inhaled before or after exercise. Furthermore, these studies demonstrate the possibility of using masks as a non-pharmacologic means of controlling EIB.