Leukotrienes (LT) are synthesized in the lung during asthmatic reactions and mediate certain inflammatory symptoms. Pulmonary metabolism and clearance of exogenously added peptidoleukotrienes were studied in nonasthmatics, asthmatics, and asthmatics challenged with allergen. [3H]LTC4 and [14C]dextran were instilled into the airways, and bronchoalveolar lavage fluid (BALF) was obtained 15 min later. After comparing the [3H]/[14C] ratio of the instilled solution with that in BALF, 77% of LT were found to have been removed from the airways of nonasthmatics, and 72% of unchallenged asthmatics. Allergen administration to asthmatics 1 min before LT instillation inhibited LT transfer out of the airways by 26%, compared with asthmatics not challenged with allergen. This decrease in the removal of LT from the airways during allergic reactions could potentiate the physiologic effects of LT produced in the airways. The predominant LT in BALF was LTE4, constituting 56% of the LT in asthmatics and 61% in nonasthmatics. The percentage of LTE4 in BALF increased to 87% in allergen-stimulated asthmatics (p < 0.05 compared with the two other groups), this again reflecting decreased transfer of LT out of the lung rather than an increase in metabolism. Urinary excretion of LT metabolites occurred rapidly, the majority being excreted within 6 h after instillation. LTE4, the major urinary LT metabolite identified by high-performance liquid chromatography, was a similar percentage concentration in the three groups and, thus, can be accurately used as an index of LT synthesis.