Fifteen subjects undertook repeated periods of hyperventilation of cold dry air and hyperventilation of warm humidified air followed by cold dry air. Hyperventilation of cold dry air induced a biphasic response with initial bronchodilatation followed by bronchoconstriction. However, exercise produced a greater rise in forced expiratory volume in 1 second (FEV1) than hyperventilation alone. Hyperventilation of cold dry air resulted in a period of refractoriness to a second stress of the same type. Refractoriness induced by exercise or hyperventilation of cold dry air may be the result of refractoriness of the trigger site in the large airways.