Because of the relative simplicity of factors governing gas exchange between the embryo and the environment, the avian embryo has been extensively studied at sea level as a model for describing the physical principles that govern the physiology of gaseous diffusion. Two patterns of response have been observed in the oxygen pressure (P(O2)) cascade of high altitude avian embryos: (1) At a fixed O2 conductance, a decrease in metabolic rate below levels typical of conspecific species at sea level results in a reduction of ΔP(O2) between ambient air and air cell. The decrease in metabolic rate also affects each step in the gradient until the mitochondria is reached. This is illustrated by a Fick's-type equation: P(O2) = V̇(O2)/G(O2) (G(O2) = oxygen conductance); (2) Maintenance of metabolic rate at roughly sea level values and a large G(O2) will also keep the ΔP(O2) unmodified or even reduced. We have observed both patterns in mountain-avian embryos in the Peruvian Andes. We measured V̇(O2) and the ΔP(O2) cascade of embryos of Peruvian coots (Fulica americana peruviana), which breed both at sea level and in the puna (Andean high altitude plateau), and those of Puna teal (Anas versicolor puna), a species that breeds solely in the puna, both at 4150 m. These two different strategies result in similar arterial P(O2) values in both values in both embryos. Because the a-v P(O2) gradient remains protected, the P(VO2) values are also similar in the embryos of both high altitude birds.