We studied the effect on airway function of chlorpheniramine maleate (C), an H1 antagonist, delivered as an aerosol to 12 asthmatic subjects, 40 minutes before submaximal exercise. The study consisted of three protocol days. A first day established that the asthmatic subjects had exercise-induced bronchospasm (EIB). On the remaining two protocol days, 10 mg of C or a saline placebo (P) were administered by aerosol nebulizer in a double-blind randomized fashion over a 5-minute period. Pulmonary function was measured immediately after the aerosol inhalation as well as 10 and 30 minutes later. A 5-7-minute period of exercise was then performed by each subject on a cycloergometer, and lung function was again measured immediately and 5 minutes after exercise. Thirty minutes after the inhalation of C there was a significant increase in lung function parameters (1.80 +/- 0.23 to 2.32 +/- 0.34/sec for MEF40%(P); Mean +/- 1 SE, P less than .05) but not for placebo. MEF40%(P) remained significantly higher on the C day than on the P day following exercise; however, this difference was no greater than the difference seen immediately before exercise. We suggest that C promotes mild bronchodilation of asthmatic airways, and that this effect in EIB may be interpreted as the result of an alteration of baseline airway caliber.