Exercise-induced asthma (EIA) was provoked by a standardized treadmill running for 8 min in seven atopic adult asthmatics. The tests were performed using a double-dummy technique after placebo, oral ketotifen, inhaled clemastine, ipratropium bromide and sodium cromoglycate (SCG), in a random single blind-fashion on different days. The mean post-exercise percentage fall in forced expiratory volume in 1 sec (FEV1) was 47 (s.e. 6.95), 39 (s.e. 8.35), 27 (s.e. 7.17), 23 (s.e. 7.69) and 7.0 (s.e. 4.62)% respectively. There was significantly less mean bronchoconstriction with SCG (P less than 0.01), ipratropium bromide and clemastine (P less than 0.05) but not with ketotifen. Six out of seven individual patients had significant protection of EIA with sodium cromoglycate, four with ipratropium bromide, three with clemastine but only one with ketotifen. Ipratropium bromide and clemastine were bronchodilators at rest, whereas SCG and ketotifen were not. Despite its claims to work as a mast cell stabilizing drug, ketotifen in a single dose does not have an effect similar to sodium cromoglycate in EIA, nor does it compare with inhaled clemastine or ipratropium bromide.