Effects of quercetin inhalation on immediate (IAR), late-phase (LAR) and late late-phase (LLAR) asthmatic responses by exposure to aerosolized-ovalbumin (AOA) (2w/v% in saline, inhalation for 3 min) were studied in conscious guinea-pigs sensitized with AOA. We measured specific airway resistance (sRaw), and recruitment of leukocytes, histamine and protein contents and phospholipase A2 (PLA2) activity in bronchoalveolar lavage fluid (BALF). Effects of quercetin (10 mg/mL, inhalation for 2 min) compared with cromolyn sodium, salbutamol, and dexamethasone inhalations, respectively. Quercetin inhalation decreased sRaw by 57.15 +/- 3.82% in IAR, 57.72 +/- 7.28% in LAR, and 55.20 +/- 5.69% in LLAR compared with AOA-inhaled control. Salbutamol inhalation (5 mg/mL) significantly inhibited sRaw in IAR, but inhalations of cromolyn sodium (10 mg/mL) and dexamethasone (5 mg/mL) significantly inhibited sRaw in IAR, LAR and LLAR, respectively. Inhibitory activity of quercetin inhalation on sRaw was similar to effect of its oral administration (10 mg/kg) in asthmatic responses. Quercetin (10 mg/mL, inhalation for 2 min) significantly decreased histamine and protein contents, PLA2 activity, and recruitments of leukocytes in BALF and also improved slightly infiltration of eosinophils and neutrophils in histopathological survey. Its anti-asthmatic activity was similar to cromolyn sodium and dexamethasone.