Late asthmatic reaction (LAR) and dual asthmatic reaction (DAR) were developed in guinea pigs by repetitive treatment with toluene 2,4-diisocyanate (TDI). Development of LAR or DAR seemed to dependent on the TDI-treatment period. IgG was demonstrated in both the LAR and DAR models, whereas IgE was only in the DAR model. Increased numbers of eosinophils and goblet cells were observed in the epithelial mucosa of the bronchioles in addition to mast cells with decreased numbers of specific granules. Both LAR and DAR were inhibited in those model animals pretreated with dexamethasone. Serum corticosteroid and adrenal cytochrome P-450 levels in guinea pigs during TDI-treatment period were similar to those in untreated animals. Also, hepatic cytochrome P-450 levels and drug metabolizing activities were similar to those in untreated animals.