The bronchodilating effect of inhaled magnesium sulfate (MgSO4) was studied in 30 asthmatic patients with mild to moderate bronchial reactivity. The patients were not in clinical remission and had a forced expiratory volume at 1 second (FEV1) lower than 80% of predicted. Subjects received, in random order, on separate days apart, saline (NaCl) and MgSO4 inhalation. Spirometry was recorded on each test day before and ten minutes after NaCl and MgSO4 inhalation. Neither NaCl nor MgSO4 was found to have a significant bronchodilating effect on spirometric measurements.