We tested the hypothesis that the inhaled tachykinin substance P (SP) can induce hyperresponsiveness to methacholine in asthmatic subjects in vivo. Nine atopic nonsmoking asthmatic males with normal forced expiratory volume in 1 s (FEV1; > 80% predicted) and increased methacholine sensitivity [provocative concn causing 20% fall in FEV1 (PC20) < 8 mg/ml] participated in a two-period placebo-controlled crossover study. Dose-response curves to SP (0.25-8 mg/ml) and placebo were recorded on 2 randomized days at least 1 wk apart, and methacholine tests were done 24 h before and 2 and 24 h after these challenges. The responses were measured by FEV1 (%fall from baseline). The position of the methacholine dose-response curves was expressed by the PC20 FEV1 and by the maximal response by the plateau level (MFEV1). SP caused a dose-dependent fall in FEV1 (P < 0.001). There was a slight increase in the PC20 FEV1 at 2 and 24 h, which was not significantly different between placebo and SP. Similarly, there was a reduction in MFEV1 at 2 h after both pretreatments. However, at 24 h after SP inhalation, MFEV1 increased compared with placebo. These changes in MFEV1 were significantly different between SP and placebo by 5.2 +/- 2.2% fall (SE) (P < 0.05). We conclude that 1) a bronchoconstrictive dose of SP, compared with placebo, enhances maximal airway narrowing to methacholine in asthma 24 h after inhalation and 2) tolerance develops to high doses of inhaled methacholine. These findings are suggestive of a role of SP in causing excessive airway narrowing in asthma by inflammatory mechanisms.