Eight young men with documented exercise-induced asthma each performed 2 standard exercise tests and underwent 2 histamine challenges on separate days after double-blind administration of 20 mg nifedipine or placebo. Exercise response was assessed by the maximal fall in forced expiratory volume in 1 s (FEV1) and to inhaled histamine by calculating the dose that produced a 20% fall in FEV1 (PC20). After premedication with nifedipine, the severity of exercise-induced asthma was significantly reduced (mean percent fall in FEV1, 6%, compared with 24% after placebo) and reactivity to histamine was reduced (geometric mean PC20, 1.68 mg .cntdot. ml-1, compared with 0.46 mg .cntdot. ml-1 after placebo). Results imply that difedipine has a direct action on bronchial smooth muscle contractility but they do not exclude an additional effect on the secretion of chemical mediators from mast cells.