It has been shown that airway exposure to eosinophil-derived cationic proteins stimulated vagal pulmonary C-fibers and markedly potentiated their responses to lung inflation in anesthetized rats (J Appl Physiol 91: 1318, 2001). However, whether the effects resulted from a direct action of these proteins on the sensory nerves was not known. The present study was therefore carried out to determine the effect of these proteins on isolated rat vagal pulmonary sensory neurons. Our results obtained from perforated whole-cell patch-clamp recordings showed that pretreatment with eosinophil major basic protein (MBP: 2 µM, 60 s) significantly increased the capsaicin-evoked inward current in these neurons; this effect peaked around 10 min after MBP and lasted for more than 60 min; in current-clamp mode, MBP substantially increased the number of action potentials evoked by both capsaicin and electrical stimulation. Pretreatment with MBP did not significantly alter the input resistance of these sensory neurons. In addition, the sensitizing effect of MBP was completely abolished when its cationic charge was neutralized by mixing with a polyanion, such as low molecular weight heparin, poly-L-glutamic or poly-L-aspartic acid, before its delivery to the neurons. Moreover, a similar sensitizing effect was also generated by other eosinophil granule-derived proteins (e.g., eosinophil peroxidase). These results demonstrate a direct, charge-dependent and long-lasting sensitizing effect of cationic proteins on pulmonary sensory neurons, which may contribute to the airway hyperresponsiveness associated with airway infiltration of eosinophils under pathophysiological conditions.