Background and objectives: Asthma is an inflammatory chronic disease that has become prevalent internationally. Melissa officinalis L. as a medicinal plant has long been used in the European and the Iranian traditional medicine for the treatment of several diseases. The biological activities, such as antioxidant, anti-tumour, antiviral, antimicrobial, and anti-inflammatory effects of M. officinalis has been reported. Therefore, the effect of Melissa officinalis L. extract on tracheal smooth muscle responsiveness, white blood cell (WBC) counts, and lung pathological changes of ovalbumin (OVA) induced asthma model rat was examined in the current study. Methods: The hydroalcoholic extract of M. officinalis was prepared using 300 g of powdered leaves. Tracheal smooth muscle responsiveness, lung pathology, and WBC counts were evaluated in control, sensitized to OVA, and sensitized rats treated with dexamethasone and three doses of M. officinalis extract (50, 150 and 200 mg/kg). Results: Tracheal smooth muscle responsiveness to methacholine hydrochloride in all sensitized groups was greater than that of the control group (p<0.001). The treatment of asthma-induced rats with dexamethasone and M. officinalis extracts (50, 100 and 200 mg/kg) remarkably reduced pathological alterations, including; inflammation, muscle hypertrophy and mucus plaques in the lung compared to the sensitized group (p<0.05 to p<0.001). Additionally, M. officinalis extract significantly improved total and differential WBC counts in broncho-alveolar lavage fluid (BALF) (p<0.001 for all groups). Conclusion: Results of the current study showed a preventive effect of M. officinalis extracts on the responsiveness of tracheal smooth muscle and lung inflammation in OVA-sensitized rats
