A double-blind randomised cross-over study was performed on 12 subjects suffering from reversible airway obstruction (asthma) to determine the relative bronchodilator effects of oral pseudoephedrine 60 mg, pseudoephedrine 180 mg, ephedrine 25 mg and matched placebo. Spirometry was used to measure vital capacity and forced expired volume in 1 s, and whole body plethysmography was used to measure specific airway conductance. Measurements were recorded before each drug was given, and 1 h and 2 h after each drug. Reversibility of the airway obstruction on each day of the study was demonstrated by significant improvements in all parameters of lung function in response to 400 micrograms of isoprenaline inhaled after the 2-h measurement. No significant bronchodilator effect could be shown following the ingestion of pseudoephedrine 60 mg or 180 mg. Only a week bronchodilator effect was demonstrated after ephedrine 25 mg in that the percentage change in vital capacity at 2 h after ephedrine was greater than that following either dose of pseudoephedrine or the placebo. It is concluded that oral pseudoephedrine in single doses of 60 mg or 180 mg has no significant bronchodilator action in man, and that a single dose of up to 180 mg pseudoephedrine does not cause tachycardia or hypertension.