Toluene diisocyanate (TDI) is a causative agent in occupational asthma. Through an oral catheter TDI, 0·03 μl, dissolved in 0·02 ml olive oil. was superfused on the tracheobronchial mucosa of anaesthetized guinea-pigs. TDI induced plasma exudation into both airway tissue and lumen (peak effect: 5 hr; duration ∼ 17 hr). Light microscopy examinations demonstrated that the epithelium was not disrupted by this process (and that microvessels are abundant just beneath the epithelium). At days 6 and 21 after exposure to TDI PAS-positive cells were increased, but no other histological alterations were found. Also, the occurrence of peptide-containing nerve fibres was not altered by TDI. After TDI-exposure the airway smooth muscle tone was elevated as examined in vitro at base-line and at concentration-response to carbachol. The largest increases in tone were recorded 21 days after exposure to TDI. The abnormally large tone was not associated with an increased thickness of the smooth muscle layer nor was it associated with reduced effects of either β2-agonist (terbutaline) or xanthine- (theophylline) relaxants. It is concluded that TDI-induced plasma exudation into guinea-pig airways occurs for 17 hr without disrupting the epithehal lining and without causing major changes in the airway peptidergic innervation. Both the airway tone, and the number of mucous cells, are increased for at least 3 weeks after exposure to TDI.