The purpose of this study is to compare the diagnostic validity (sensitivity and specificity) and evaluate the safety of 2-min Tidal Breathing methacholine bronchial provocation tests with the Wright and Devilbiss646 nebulizers in asthmatic in Guangzhou.

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.

Inclusion Criteria:

• clinical diagnosis of asthma;
• stable for more than 4 weeks with FEV1≥65%pred

Exclusion Criteria:

• exacerbation within 3 months;
• upper airway infections within 4 weeks;
• concomitant of hypertension or heart diseases;
• pregnancy;
• under immunotherapy