The chemoreflex control of breathing is described in terms of a graphical model. The central chemoreflex, the ventilatory response to carbon dioxide mediated by the central chemoreceptors, is modelled as a straight-line relation between the ventilatory response and the arterial level of carbon dioxide. The peripheral chemoreflex, the ventilatory response to carbon dioxide and hypoxia mediated by the peripheral chemoreceptors, is broken into two relations. First, a straight-line relation between the ventilatory response and the arterial level of carbon dioxide whose slope (sensitivity) increases as the oxygen level varies from hyperoxic to hypoxic. Second, a rectangular hyperbolic relation between the ventilatory response and the arterial level of oxygen with ventilation increasing with increasing hypoxia. The three ventilatory response relations (one central and two peripheral) add to produce the total chemoreflex ventilatory response which forms the feedback part of the respiratory regulator. The forward part consists of the relation between the arterial level of carbon dioxide and ventilation when ventilation is controlled (the metabolic hyperbola). The forward and feedback parts of the respiratory regulator can be combined so as to predict resting ventilation and carbon dioxide levels under a number of circumstances. Methods of measurement of these chemoreflex ventilatory responses are also described so as to illustrate the physiological principles involved in the model.