Guinea-pig isolated lungs were perfused at constant flow rate via the, trachea with Krebs saline. Extracts were injected into the perfusion fluid and bronchoconstriction was assessed as a rise of perfusion pressure. Extracts of cotton dust and bract caused a dose-related reversible bronchoconstriction that was unaffected by mepyramine. In this preparation endotoxin exhibited a bronchodilator effect. Methysergide and indomethacin each partially antagonized the effect of bracts but not if dust. However, FPL-55712 antagonized the response to cotton dust but not bracts. The results suggest that the responses to tried extracts are mediated partly by release of arachidonic acid metabolites of the leukotriene and prostaglandin pathways for dust and bracts respectively. Examination of the perfusate confirmed that cotton dust released arachidonate metabolites in addition to histamine.