Human asthma is characterized by three critical phenotypic traits: intermittent reversible airway obstruction, airway hyperresponsiveness and airway inflammation. In animal models of asthma, airway hyperresponsiveness is an important feature. This trait is characterized by an exaggerated bronchoconstrictor response that would have little physiological consequence in an otherwise unaffected or normal individual. In this article we explore two distinct facets of airway responsiveness. The first is the genetic basis for variations in airway responsiveness that occur in mice in the absence of any specific environmental manipulation. We demonstrate that standard genetic approaches can be successfully applied to the identification of regions of the mouse genome linked to the expression of airway hyperresponsiveness. The second topic addressed in this review is the change in airway responsiveness induced in rats by repeated exposure to sulphur dioxide gas. With daily exposure to high concentrations of sulphur dioxide gas, there is chronic injury and repair of epithelial cells. Over time, rats develop mucous hypersecretion, airway inflammation, increased airway resistance and airway hyperresponsiveness. This model has provided useful information on the mechanisms underlying the pathophysiological events that typify the chronic bronchitis in humans.