The role of small airways in the immediate allergic response is largely unknown. We therefore used the model of precision-cut lung slices (PCLS) in combination with quantitative videomicroscopy to study the early allergic response to allergen in airways ranging from 50 to 900 microm. After PCLS from untreated Wistar rats had been passively sensitized for 16 h with serum from sensitized Brown Norway rats, exposure to 0.1% ovalbumin resulted in an immediate allergic response. Both extent (r = 0.74, p < 0.0001) and velocity (r = 0.49, p < 0.0001) of the allergen-induced bronchoconstriction increased with decreasing airway size. In addition, we observed that smaller airways not only contracted stronger and quicker, but that they also relaxed faster, suggesting that smaller airways are more reactive in principle. The allergen-induced bronchoconstriction in PCLS was prevented by the serotonin receptor antagonist ketanserin (IC(50) 6 nM), but not by antagonists directed against histamine, acetylcholine, PAF, or endothelin receptors, or by cyclooxygenase or lipoxygenase inhibitors. Like allergen, serotonin provoked responses that were stronger in smaller airways. These findings suggest that the immediate allergic response in rat PCLS depends largely on serotonin and that this response can occur in nearly all airway generations, but is most pronounced in the smallest airways, that is, the terminal bronchioles.