We studied acute effects of tumor necrosis factor-alpha (TNFα) on the sensitivity of isolated rat vagal pulmonary sensory neurons. Our results showed: 1) A brief pretreatment with a low dose of TNFα (1.44 nM, 9 min) enhanced the sensitivity of transient receptor potential vanilloid type 1 (TRPV1) receptors in these neurons in two distinct phases: the inward current evoked by capsaicin was amplified (=247%) immediately following the TNFα pretreatment, which gradually declined toward control and then increased again reaching another peak (=384%) after 60-90 min. 2) The immediate phase of this potentiating effect of TNFα was completely abolished by a pretreatment with a selective cyclooxygenase-2 (COX-2) inhibitor, NS-398, whereas the delayed potentiation was only partially attenuated. 3) In sharp contrast, TNFα did not generate any potentiating effect on the responses to non-TRPV1 chemical activators of these neurons. 4) The selectivity of the TNFα action on TRPV1 was further illustrated by the responses to acid (pH 6.0); TNFα did not affect the rapid transient current mediated by acid-sensing ion channels, but significantly augmented the slow sustained current mediated by TRPV1 in the same neurons. 5) In anesthetized rats, a similar pattern of acute sensitizing effects of TNFα on pulmonary C-fiber afferents and the involvement of COX-2 were also clearly shown. In conclusion, a brief pretreatment with TNFα induced both immediate and delayed potentiating effects on the TRPV1 sensitivity in pulmonary sensory neurons, and the production of COX-2 arachidonic acid metabolites plays a major role in the immediate sensitizing effect of TNFα.