Airway hyperresponsiveness develops in dogs after ozone inhalation. This study examined the role of the parasympathetic nervous system in ozone-induced airway hyperresponsiveness in dogs. Dose-response curves to acetylcholine (n = 8) and histamine (n = 4) were measured before and after exposure to ozone (3 ppm for 30 min). The provocative concentration of each agonist was measured on two randomly assigned days separated by at least 1 wk. On one day a control experiment was performed, and on the other day the dogs were pretreated with the ganglionic blocker hexamethonium bromide in doses that block ganglionic transmission. The acetylcholine provocative concentration decreased on the control day from 5.5 mg/ml (%SE 1.8) before ozone to 0.5 mg/ml (%SE 2.0) after ozone (P less than 0.0001). After pretreatment with hexamethonium the acetylcholine provocative concentration decreased from 9.0 mg/ml (%SE 1.8) before ozone to 1.0 mg/ml (%SE 2.0) after ozone (P = 0.002). The results were similar when histamine was used as the agonist. Therefore, ganglionic blockade does not prevent airway hyperresponsiveness after ozone inhalation, and a parasympathetic reflex mechanism is not responsible for airway hyperresponsiveness after ozone inhalation in dogs.