The sensitized guinea pig was employed as a model to study the effect of immunological activation of resident mast cells on neuronal activity in the airways. The trachea was isolated with the vagus nerves and vagal sensory ganglia intact. Using conventional electrophysiological recording techniques, we noted that antigenic stimulation led to an increase in the sensitivity of sensory nerve endings located in the airway wall. Moreover, previous work has revealed that antigen challenge potentiates action-potential-driven tachykinin release from afferent fibers in the guinea pig isolated airway. Anatomical and electrophysiological studies indicate that tachykinin-containing sensory fibers directly innervate the local parasympathetic ganglion neurons in the airway. Therefore, antigen-induced increases in the excitability of sensory fibers in the airways can increase parasympathetic tone in the airway by increasing central and peripheral reflex arcs. In addition, we have found that antigen stimulation has direct effects on the excitability of the parasympathetic ganglion neurons. Considered together, the data demonstrate that increased vagal sensory and parasympathetic activity may be a consequence of the allergic reaction in the airways.