It has been suggested that halothane inhibits contraction of airway smooth muscle in vivo mainly by reducing reflex activity in nerves innervating the muscle with only minimal direct effects on the muscle itself. To examine possible mechanisms of action of halothane at clinically relevant concentrations the authors studied the effect of halothane on increases in pulmonary resistance (RL) produced by either vagus nerve stimulation (VNS, which caused neurally mediated constriction) or the inhalation of nebulized acetylcholine (ACh, which directly stimulated the smooth muscle cell) in nine mongrel dogs. The frequency of bilateral VNS and the dose of nebulized ACh were adjusted to produce approximately equal increases in RL. Halothane reduced the response to both types of stimulation in a dose-dependent fashion. At halothane concentrations greater than or equal to 0.4 MAC, the VNS response was significantly less than the ACh response. When tetrodotoxin was given to block neural activity, the ACh response was unchanged, confirming that neural activation did not contribute significantly to smooth muscle contraction in response to ACh. The authors conclude that in addition to neurally mediated effects, halothane at clinically used concentrations has significant direct effects on airway smooth muscle stimulated by ACh. The relative importance of each factor in vivo should depend on the stimulus that causes contraction of airway smooth muscle.