Major basic protein and other native cationic proteins increase airway hyperresponsiveness when administered to the luminal surface of the airways in vitro. To determine whether the same applies in vivo, we assessed airway responsiveness in rats challenged with both aerosolized and intravenously infused methacholine. We partitioned total lung resistance into its airway and tissue components using the alveolar capsule technique. Neither poly-l-lysine nor major basic protein altered baseline mechanics or its dependence on positive end-expiratory pressures ranging from 1 to 13 cmH(2)O. When methacholine was administered to the lungs as an aerosol, both cationic proteins increased responsiveness as measured by airway resistance, tissue resistance, and tissue elastance. However, responsiveness of all three parameters was unchanged when the methacholine was infused. Together, these findings suggest that cationic proteins alter airway responsiveness in vivo by an effect that is apparently limited to the bronchial epithelium.