We have previously shown that tachykinin depletion or antagonism prevented the increase in airway responsiveness to inhaled acetylcholine caused by exposure to toluene diisocyanate (TDI) in awake guinea pigs. To insure that the effects of tachykinins were not limited to the extrathoracic airways and were not dependent on effects of TDI on baseline airway caliber, we determined airway responsiveness to acetylcholine inhaled through a tracheostomy in anesthetized and ventilated guinea pigs that were exposed to TDI or air after treatment with the tachykinin antagonist spantide, the tachykinin metabolism inhibitor phosphoramidon, or the vehicles for each drug. When these drugs were administered before and during TDI exposure, spantide significantly inhibited the TDI-induced increase in acetylcholine responsiveness and phosphoramidon significantly potentiated this effect, whereas neither drug altered acetylcholine responsiveness in air-exposed animals. To determine whether tachykinins were exerting their effect primarily during TDI exposure or during the subsequent acetylcholine challenge, we also examined the effect of each drug on acetylcholine responsiveness when the drugs were given after TDI exposure. At that time, spantide did not inhibit TDI-induced acetylcholine hyperresponsiveness and phosphoramidon did not potentiate it. Neither drug nor TDI increased pulmonary resistance measured through a tracheostomy in these anesthetized and ventilated animals. These results suggest that the TDI-induced increase in acetylcholine responsiveness is mediated by release of tachykinins into the intrathoracic airways during exposure to TDI.