Although changes in airway responsiveness in pulmonary inflammation are commonly related to the action of infiltrated leukocytes, our previous report suggested a direct role of inflammatory cytokines in LPS-induced lung hyporesponsiveness. The aim of this study was to define if cytokines detected in the BALF (bronchoalveolar lavage fluid) of intratracheal LPS-treated mice could be, at least in part, responsible for 5-HT (5-hydroxytryptamine) lung hyporeactivity. Our results show that intratracheal instillation of LPS induced a time-dependent increase in IL-(interleukin-)1beta, IL-6, and TNF (tumor necrosis factor)alpha in the BALF. Cytokine production was paralleled by 5-HT lung hyporesponsiveness, and intratracheal administration of TNFalpha proved to be very efficient in inhibiting 5-HT responsiveness. In addition, systemic treatment with rolipram, an inhibitor of TNFalpha production, was paralleled by a significant recovery of lung responsiveness. On the contrary, IL-1beta and IL-6 were not demonstrated to play a relevant role in 5-HT hyporesponsiveness. It is concluded that TNFalpha could be a crucial mediator of LPS-induced lung hyporesponsiveness.