Recent studies have demonstrated that the calcium channel blocking agents can inhibit experimentally induced bronchoconstriction in asthmatics, but their protective action has been variable. To clarify the influence of stimulus intensity and choice of calcium blocker on these reported differences in outcome, we performed noncumulative thermal stimulus-response curves using isocapnic hyperventilation of cold air in 8 asthmatics. Subjects received pretreatment with orally administered nifedipine (20 mg), intravenously administered verapamil (10 mg bolus followed by a continuous infusion), or appropriate placebos in a randomized, double-blind fashion. Verapamil afforded no consistent protection against the thermal challenges, whereas nifedipine significantly blunted the bronchoconstrictor response to stimuli of low (p less than 0.02) and middle (p less than 0.03) intensity. At the highest thermal burden, the effect of nifedipine was inconsistent and not significantly different from that of placebo. These results indicate that the protection from bronchoconstriction afforded by the calcium channel blockers depends on the choice of agent and the intensity of the bronchoconstricting stimulus, and they raise the possibility that the contribution of transmembrane calcium ion influx to the pathogenesis of bronchoconstriction may vary according to stimulus intensity.