The main aim of the present study was to evaluate whether inhaled salmeterol given in the evening protected against exercise-induced asthma the next morning. Twenty three children (12 males and 11 females) with a mean age of 11 yrs and with exercise-induced asthma participated in a double-blind, randomized, placebo-controlled study. The children inhaled salmeterol 25 micrograms, salmeterol 50 micrograms and placebo by Diskhaler at 10 p.m. on 3 separate days. Next morning, half of the children ran on a motor-driven treadmill for 6 min at submaximal load at 8 a.m. and the remainder at 10 a.m. Lung function was measured by maximal expiratory flow-volume loops before running, immediately after, and 3, 6, 10 and 15 min after running. The mean maximum reduction in forced expiratory volume in one second (FEV1) after treadmill run was 34% before inclusion in the study. Mean maximum fall in FEV1 was significantly greater after placebo: 30% (23-36) 95% confidence interval) than after salmeterol 25 micrograms: 19% (12-23) or salmeterol 50 micrograms: 18% (12-25). In addition to the reduced postexercise bronchoconstriction, pre-exercise lung function (FEV1) was significantly higher both after salmeterol 25 micrograms: 2.4 L.s-1 (2.1-2.7) and salmeterol 50 micrograms: 2.5 L.s-1 (2.2-2.8) than after placebo: 2.2 L.s-1 (1.9-2.5). No significant differences in pre- and postexercise lung function were found between children tested at 8 or 10 a.m., or in relation to salmeterol dosage. Thus, inhaled salmeterol 25 and 50 micrograms offered similar overnight protection against exercise-induced asthma and improved baseline lung function in the morning as compared to placebo.