The effects of inhaled MgSO4 on histamine bronchoprovocation test (BPT) were studied in nine asthmatics in clinical remission (FEV1 greater than 80% of predicted). Patients performed histamine BPT on 2 separate days, one day after saline and the other after MgSO4 inhalation, in a randomized double-blind design. Spirometry and flow/volume curve were recorded on each test day before and 5 min after NaCl or MgSO4. No significant difference was observed in lung function measurements 2 days before and after either NaCl or MgSO4. The dose of histamine which produced a 20% decrease in control FEV1 (PD20FEV1) was significantly increased by aerosolized MgSO4 (from 0.177 +/- 0.036 mg after NaCl to 0.350 +/- 0.085 after MgSO4, P less than 0.05. After MgSO4 the dose-steps of histamine concentration increased two-fold in two subjects and one-fold in five.