Since the optimal dose of salmeterol in asthmatic children has not yet been clearly defined, we compared the efficacy and duration of the protective effect of two doses of salmeterol (25 and 50 micrograms) against exercise-induced bronchoconstriction. Twelve children (aged 7-14 yrs) with asthma were studied in a double-blind, cross-over, placebo-controlled design. On three separate days, exercise tests were performed 1 h and 12 h after administration of the drug. Pulmonary function measurements were performed before drug inhalation, before every exercise test and 1, 5, 10, 15 and 30 min after the end of exercise. The response was expressed as maximal decrease in forced expiratory volume in one second (FEV1). Both doses of salmeterol provided significant bronchodilation for up to 12 h, with no difference between them. Maximal exercise-induced decrease in FEV1 (% fall) 1 h after pretreatment was (mean +/- SD) 35 +/- 16, 10 +/- 10 and 4 +/- 3% for placebo, 25 and 50 micrograms salmeterol, respectively. At 12 h after pretreatment these values were 31 +/- 14, 19 +/- 12 and 15 +/- 13%, respectively. Individual protection against exercise-induced bronchoconstriction at 1 and 12 h did not vary between the dosages (p < 0.05), even though the protection obtained by 25 micrograms at 12 h was no longer significant versus placebo. We conclude that 25 micrograms of inhaled salmeterol provides equally effective long-lasting bronchodilation and acute protection against exercise-induced bronchoconstriction as 50 micrograms, and may be a suitable dose for most asthmatic children.