The effects of 800 micrograms of inhaled SK&F 104353, a peptidoleukotriene receptor antagonist, and of 20 mg disodium cromoglycate (DSCG) on exercise-induced bronchoconstriction were compared in 18 asthmatic patients. The study was conducted according to a double-blind, crossover, randomized, placebo-controlled design. Two baseline exercise tests were carried out, and pulmonary function tests were done before and at 1, 5, 10, 15, 20, and 30 min after completion of the exercise. Patients showing a 20% or greater decrease in FEV1 in both exercise challenges entered the blinded portion of the study. When placebo was administered before exercise, FEV1 fell to the same extent as during the baseline phase. After SK&F 104353 and DSCG, the bronchoconstriction was attenuated. The mean maximal percentage fall in FEV1 after exercise was 29% after placebo and 20% after SK&F 104353 and DSCG. The differences between the two active treatments did not reach the 5% level of statistical significance, though at 20 min SK&F 104353 showed a more pronounced effect than DSCG. The protective effect suggests an important role of leukotrienes in the pathogenesis of exercise-induced bronchoconstriction.