Sch 37224 is an experimental antiallergy coumpound that inhibits hyperventilation-induced bronchoconstriction (HIB) in guinea pigs and cold air bronchospasm in human asthmatics. HIB in guinea pigs may involve the release of tachykinins, such as neurokinin A (NKA) and substance P (SP), and the action of Sch 37224 in this model may relate to inhibition of these neuropeptides. We studied the effect of Sch 37224 on the neuropeptide component of HIB that was enhanced in guinea pigs treated with the neutral endopeptidase inhibitors, thiorphan and phosphoramidon. Pulmonary resistance (R_L) and dynamic lung compliance (C_oyn) were measured in anesthetized, mechanically ventilated guinea pigs. R_L and C_Dyn were measured at baseline (1 ml/l00 g tidal volume and 50 breaths/min) and after a 10-min period of hyperventilation (1 ml/l00 g, 150 breaths/min). Hyperventilation produced modest changes in R_L (+41 ± 12%) and C_oyn (–12 ± 3%) which were markedly enhanced by treatment with 3 mg/kg of either thiorphan or phosphoramidon (Rl + 269 ± 43% for thiorphan, + 292 ± 63% for phosphoramidon and C_oyn –65 ± 3% for thiorphan, -51 ± 13% for phosphoramidon). In the presence of thiorphan or phosphoramidon, the bronchospasm to hyperventilation was significantly reduced (>70%) with 5 mg/kg, p.o., of Sch 37224. In other studies, the peptidergic (conducted in the presence of ipratropium bromide and phosphoramidon) bronchoconstrictor response to intravenous nicotine (1 mg/kg) was also inhibited by Sch 37224 (0.3-10 mg/kg, p.o.). However, Sch 37224 (5 mg/kg, p.o.) had no effect on the bronchoconstrictor response to intravenous NKA. These results indicate that Sch 37224 inhibits the neuropeptide component of HIB and nicotine in guinea pigs and this effect appears to be mediated by the inhibition of the release of tachykinins from airway C fibers.