Lung parenchymal strips (LPS) of horse plasma-sensitized and control sheep were studied isometrically in isolated organ baths and their responses to autacoid and autonomic agents were compared. Control LPS responded with contractions to histamine greater than carbachol greater than 5HT and relaxed to isoproterenol greater than adrenaline greater than phenylephrine. In sensitized LPS, adrenaline and phenylephrine-induced relaxations were converted to contractions and a new order of potency and efficacy for spasmogens was observed i.e. histamine greater than carbachol greater than adrenaline greater than phenylephrine greater than 5HT. Isoproterenol was also significantly (P less than 0.05) less potent and less effective in relaxing sensitized LPS compared to controls. Mepyramine, atropine and propranolol competitively antagonized their respective agonists (i.e. histamine, carbachol and isoproterenol) confirming the presence of H1-histaminergic, muscarinic-cholinergic and beta-adrenergic receptors respectively. The conversion of the relaxant effect of adrenaline and phenylephrine to a contraction supplemented by a significant reduction in isoproterenol activity suggests an impairment in the effective ratio of beta: alpha adrenoceptors in ovine LPS subsequent to antigenic sensitization.