A plethysmographic method was employed to assess the airway resistance of conscious, free-breathing guinea-pigs. Using this method animals sensitized by inhalation of ovalbumin and appropriate controls were assessed for their responsiveness to histamine and methacholine in vivo. The cough frequency on exposure to citric acid mist in the two groups was also assessed. Tracheal spirals from these animals were subsequently tested for their responsiveness to histamine, methacholine and prostaglandin D2 in vitro. Sensitization increased responsiveness to histamine, methacholine and citric acid in vivo but only histamine responses were affected in vitro. These changes were accompanied by a significant eosinophilia in the airways as assessed by bronchoalveolar lavage. We conclude that sensitization of the airways to ovalbumin results in responsiveness changes in bronchial smooth muscle accompanied by signs of airway inflammation.