As ectothermic vertebrates, reptiles have low oxygen demands, yet they appear to be endowed with all of the mechanism for oxygen sensing that have been described in mammals. Thus, when faced with hypoxia or reductions in blood oxygen carrying capacity, reptiles respond by increasing cardiac output and ventilation. In addition, reptiles can reduce the magnitude of the intracardiac shunt to elevate blood oxygen delivery, and they may lower the demand for oxygen by selecting lower body temperatures. The signals mediating these responses have been studied through independent manipulations of blood oxygen concentrations and partial pressures. These studies show that the ventilatory responses to hypoxia are solely mediated by reduction in oxygen partial pressures, while the cardiovascular responses may also be influenced by reductions in blood oxygen concentrations. The pulmonary vasculature of many species of reptiles contracts in response to hypoxia and may serve to improve the local matching of ventilation and perfusion, but the increased pulmonary vascular resistance also acts to induce a right to left shunt that may have detrimental effects on oxygen delivery.