Procaterol and albuterol, beta agonists, were studied using a placebo-controlled, repeated exercise challenge design in order to assess their duration of effectiveness in both bronchodilation and in modifying exercise-induced asthma (EIA). Fifty-three subjects aged 12 to 50 years who had at least a 20% drop in FEV1 during a screening exercise tolerance test were entered. Subjects took two inhalations of procaterol (10 micrograms/inhalation), albuterol (90 micrograms/inhalation), or placebo. Thirty minutes later they exercised on a treadmill at a workload sufficient to induce greater than or equal to 80% aerobic O2 consumption for six minutes. Pulmonary function was measured before and serially for 30 minutes after exercise. The same exercise challenge was repeated three, six, and nine hours after drug administration. Both procaterol and albuterol bronchodilated and modified EIA at 30 minutes and three hours, mean drops in FEV1 being 8.2 and 9.7% respectively at 30 minutes and 16.8 and 16.3% at three hours. This was compared with placebo falls of 30% and 26%. At six hours the subjects' response was similar after both procaterol and albuterol, and fewer subjects had a 20% fall in FEV1 than with placebo, although protection afforded by both beta agonists was substantially less than at three hours. Both drugs were tolerated well.