Salivary pH is associated with esophageal acid reflux and neutralization of esophageal acid. In this study, we assessed the association between nocturnal decline of salivary pH and airway hyperresponsiveness. Salivary pH was serially assessed in 9 patients with mild asthma (7 men and 2 women; mean age 33.3 years; mean %predicted FEV(1.0) 89.4%) and 10 healthy volunteers (6 men and 4 women; mean age 31.2 years) using a pH indicator tape. The buffering capacity of saliva was defined as the median effective dose (ED(50)) for acidification of saliva with 0.01 N HCl, and airway responsiveness was defined as the dose of methacholine producing a 35% fall in Grs (PD(35)-Grs). There was a significant correlation between the values obtained from the pH indicator tape and those obtained from the electrometric pH meter. Using the indicator tape for sequential monitoring, we observed a nocturnal fall (ΔpH) in salivary pH in all subjects. A significant correlation was found between airway hyperresponsiveness (PD(35)-Grs) and either ΔpH or ED(50) in mildly asthmatic patients. Vagal reflux dysfunction might contribute to nocturnal salivary pH as well as to airway hyperresponsiveness in mild asthmatics.