1 In a preliminary double-blind trial the effects of ephedrine and pseudoephedrine on the blood pressure and heart rate of resting healthy volunteers were compared. Ephedrine 60 or 90 mg were required to raise the diastolic blood pressure above 90 mmHg, whereas 210 or 240 mg pseudoephedrine were required to produce the same effect. 2 In a second double-blind trial, patients with reversible airways obstruction were given 60 mg ephedrine or 210 mg pseudoephedrine and the effects on forced expiratory volume in one second (FEV1) compared. Both isomers produced some bronchodilation, but the effect of pseudoephedrine was less than half that of ephedrine. 3 The reasons for these differences between the isomers are discussed and the efficacy of pseudoephedrine as a nasal decongestant pointed out and explained in relation to its effect on alpha-adrenoceptors in the nasal blood vessels.