Seven asthmatic children with moderately to severely increased bronchial responsiveness to methacholine took part in a double-blind placebo-controlled study to assess the effect of a calcium channel blocker, diltiazem, on exercise-induced asthma (EIA), and its duration. On the control day, bronchial response to exercise was found to be highly reproducible when performed 2 h apart (intraclass correlation coefficient 0.92). Normal saline and diltiazem at concentrations of 1.75 and 3.50 mg/ml (estimated nebulized doses 5 and 10 mg, respectively) were given in random order before exercise on a bicycle ergometer. Exercise challenge was performed 20 min and 2 h after each treatment, and bronchial response was expressed as percent fall in the forced expiratory volume in 1 s. In the overall group, diltiazem did not significantly change resting bronchial tone and produced no significant (p = 0.18) attenuation of EIA. An almost complete protection was detected only in two subjects 20 min after diltiazem 10 mg. This limited effect waned 2 h after the administration.