A double-blind controlled exercise challenge study has been performed in 16 asthmatic chlidren to show the effectiveness of BRL10833 in inhibiting exercise-induced bronchoconstriction. The children attended the respiratory laboratory on four occasions within the space of two weeks; on each occassion a routine 6-min exercise test was performed. At the first visit no drugs were given before the exercise test and all the children demonstrated abnormal exercise-induced bronchoconstriction as measured by peak expiratory flow rate (PEFR), forced expiratory volume in 1 sec (FEV1) and forced vital capacity (FVC). On the other three occasions the children were given sodium cromoglycate, BRL10833 or placebo medications before the exercise test. After sodium cromoglycate administration four children showed complete blocking and four showed partial blocking of exercise-induced bronchoconstriction. After BRL10833 four children showed complete blocking and six showed partial blocking of exercise induced bronchoconstriction. Placebo administration produced complete blocking of exercise-induced bronchoconstriction in three and partial blocking in two children. The results indicated that BRL10833 was almost as effective as sodium cromoglycate in inhibiting exercise-induced bronchoconstriction and placebo, although less effective than the two preparations, did afford protection from exercise-induced bronchoconstriction in some of the children.