In a randomized, double-blind, placebo controlled cross-over study we have investigated the effect of intravenous magnesium on airway calibre and airway reactivity to histamine in 20 subjects with mild to moderate asthma. After baseline measurements of forced expiratory volume in one second (FEV1), subjects received 100 ml normal saline with or without 2 g of magnesium sulphate by infusion over 20 min. Measurements of FEV1 were repeated at 5 min intervals throughout the infusion, and the provocative dose of histamine required to drop the FEV1 by 20% from baseline (PD20FEV1) was determined at 20 min. The area under the curve (AUC) in litre minutes for change from baseline in FEV1 between 0 and 20 min was significantly higher on the magnesium study day (mean difference in AUC (95% CI) 1.71 (0.02-3.4), P = 0.049). The increase in FEV1 from baseline with magnesium relative to saline was maximal at 20 min (mean difference (95% CI) 0.13 (0.02-0.23) l, P = 0.01). Log PD20FEV1 to histamine was not significantly different after magnesium and saline (mean difference in log PD20FEV1 (95% CI) 0.04 (-0.19 to 0.27), P = 0.7). We conclude that intravenous magnesium is a weak bronchodilator but does not alter airway reactivity at this dose in stable asthmatic subjects.