After screening,each asthmatic patient will visit the investigators' center 4 times.
During the first week, the patient will be randomised to undergo methacholine bronchial challenge with Wright or Devilbiss 646 nebulizer on two separate days, which are at least 24 hours but not more than 7 days apart.The above procedure will be repeated for each patient in one month. But the nebulizer will be selected in a reverse order.Provocative concentration induced a 20% decrease in forced expiratory volume in one second (PC20) will be calculated for each test. Record all the adverse events during the tests.
