The bronchodilating effect of magnesium sulfate (MgSO4) was studied in ten asthmatic patients with moderate to severe airway obstruction. Two grams of MgSO4 or saline in double-blind crossover design was administered IV for 20 minutes (0.40 mmol/min) and forced expiratory capacity and forced expiratory volume in one second (FEV1) were studied at intervals. Only at the end of MgSO4 infusion did FEV1 increase significantly (109% of initial values). The bronchodilating effect was short lasting and far less than that observed after salbutamol.