Exhaled nitric oxide (NO) is an approved marker of asthmatic airway inflammation but its role in bronchoconstriction remains yet unclear. We have evaluated the effect of NO in antigen-induced contractions of rat precision-cut lung slices (PCLS). The major mediator of the antigen-induced response in this in vitro model is known to be serotonin (Martin, C et al. AJRCCM 2001; 163:1462-1469). PCLS were passively sensitised over night with 1% serum from rats sensitised to the allergen ovalbumin (OVA). The airways were imaged and digitised with a digital video camera. Airway area before addition of any drugs was defined as 100%. Contractions were expressed as percentage decrease in airway area compared to initial airway area. Pretreatment with NO synthesis inhibitor Nω-Nitro-L-arginine Methyl Ester (L-NAME, 100 μM) enhanced the contractile response to cumulative challenge with serotonin (0.01-10 μM); 56±10.7% vs. control 84±9.3% at 1 μM serotonin (n=5, p<0.05). The contractile response to cumulative challenge with OVA (0.001-1000 mg/L) were not affected by L-NAME 100 μM (n=5), whereas pretreatment with the NO-donor NCX-2057 (3-(4-Hydroxy-3-methoxyphenyl)-2-propenoic acid) 4-(nitrooxy)butyl ester, 100 μM) reduced the response; 60±11.2% vs. control 34±4.6% at 1 mg/L OVA (n=4, p<0.05). Our data indicate that NO has a protective role against bronchoconstriction in rat PCLS. Since L-NAME enhanced the contractile response to serotonin but not to OVA, antigen-induced contractions of rat PCLS may involve other modifying mediators or actions on different levels.