Low oxygen levels in the lungs of birds and mammals cause constriction of the pulmonary vasculature that elevates resistance to pulmonary blood flow and increase pulmonary blood pressure. This hypoxic pulmonary vasoconstriction (HPV) diverts pulmonary blood flow from poorly ventilated and hypoxic areas of the lung to more well-ventilated parts and is considered important for the local matching of ventilation to blood perfusion. In the present study, the effects of acute hypoxia on pulmonary and systemic blood flows and pressures were measured in four species of anesthetized reptiles with diverse lung structures and heart morphologies: Varanid lizards (Varanus exanthematicus), caimans (Caiman latirostris), rattlesnakes (Crotalus durissus) and tegu lizards (Tupinambis merianae). As previously shown in turtles, hypoxia caused a reversible constriction of the pulmonary vasculature in varanids and caimans decreasing pulmonary vascular conductance (G_pul) by 37% and 31%, respectively. These three species possess complex multicameral lungs and it is likely that HPV would aid to secure V/Q homogeneity. There was no HPV in rattlesnakes that have structurally simple lungs where local V/Q inhomogeneities are less likely to occur. However, tegu lizards that also have simple unicameral lungs did exhibit HPV decreasing G_pul by 32%, albeit at a lower threshold than varanids and caimans (6.2 kPa oxygen in inspired air vs. 8.2 and 13.9kPa, respectively). Thus, while these observations suggests that HPV is more pronounced in species with complex lungs and functionally divided hearts, it is also clear that other components are involved.