Exogenously applied tachykinins produce no measurable electrophysiological responses in the somata of vagal afferent neurons (nodose ganglion neurons, NGNs) isolated from naive guinea pigs. By contrast, following in vitro antigen challenge of nodose ganglia from guinea pigs immunized with chick ovalbumin, ~60% (53 of 89 )of NGNs were depolarized an average of 13 ± 1.2 mV by Substance P (SP, 100 nM; n=53). Receptor antagonists and enzyme inhibitors were utilized to screen a number of mast cell-derived mediators for their role in the uncovering or 'unmasking' of functional tachykinin receptors following antigen challenge. Two chemically distinct 5-HT₃ receptor antagonists significantly reduced the percentage of NGNs displaying depolarizing SP responses. Treatment with Y-25130 (1 or 10 µM) or tropisetron (1 µM) 15 min before and during antigen challenge reduced the percentage of SP responsive neurons to ~20% and ~15%, respectively. These results suggest that activation of 5-HT₃ receptors plays an integral role in the unmasking of functional tachykinin receptors following specific antigen challenge of nodose ganglia. The mediator(s) underlying tachykinin receptor unmasking in the remainder of the NGNs has yet to be characterized. However, it does not appear to be histamine, prostanoids or peptidoleukotrienes.