The immediate changes in regional ventilation and pulmonary blood flow were studied in fifteen adults with perennial asthma and in two control persons, who were challenged by moderate exercise on a treadmill or bicycle ergometer. Ventilation was studied before and immediately after the exercise, and again about five min after stopping the provocation. Peak expiratory flow was checked at regular intervals. Five min after cessation of the exercise ventilation had become patchy and irregular, but the pulmonary blood flow distribution remained "normal", i.e. as seen in the resting state. The ventilatory defects could be prevented by pretreatment with terbutaline or eliminated by terbutaline treatment after development of signs and symptoms. From repeat studies it can be concluded that the development of regional ventilatory defects after EIA do not follow a stereotypic course nor are defects similar to those provoked by histamine or allergens.