 Salbutamol on prn basis will allow throughout the study except that it will be stopped 6 hours prior to Visit 3, 4, 5, 6 and 7. Long-acting 2-agonist will be prohibited after Visit 1 and throughout study duration. Inhaled corticosteroids will be allowed at the same dosage as before the study. PDE4 inhibitors and leukotriene antagonists will also be allowed. 