Efficacy of β2-agonists (B2), anticholinergics, corticosteroids, anti-inflammatory drugs or antihistamines against methacholine, histamine, or allergen-induced lung constriction was tested in ovalbumin sensitized guinea pigs. Airway resistance (Raw) and parenchymal damping (G) and elastance (H) were determined from low-frequency forced oscillatory input impedance (ZL). ZL was measured under control conditions, during iv infusions of methacholine, and following iv boluses of histamine or ovalbumin. In decreasing potency, ipratropium, salmeterol, fluticasone or cromoglycate reduced the methacholine-induced increases in Raw and G. Only antihistamines had any effects on the histamine-induced increased in Raw, G, and H. The ovalbumin-induced increases in Raw, G and H in the control animals (120±90%, 201±126%, 86±71%) were markedly reduced by pretreatments with antihistamines (18±11%, 13±9%, 3±3%) and cromoglycate (29±13%, 35±22%, 18±10%). Bronchoconstriction induced by muscarinic receptor stimulation is inhibited by anticholinergic, anti-inflammatory or β2-agonist pretreatment; antihistaminic or anti-inflammatory premedication is beneficial if the release of histamine via an exposure to allergen is responsible for the fall in lung function.