At the end of incubation, the partial pressures of oxygen and carbon dioxide in the air cell of sea-level avian eggs are similar to those in the expiratory air of adult birds. At high altitude, changes in the permeability of the shell and probably in the embryo metabolism partially compensates the increase in the gas diffusion constant resulting from the low barometric pressure. The aim of this study was to test whether--despite of the adaptive responses of the high altitude avian embryo--the air cell values would be similar to those of the alveolar air of high altitude human natives. Air cell O2 (48.3 +/- 1.6 torr) and CO2 (20.9 +/- 0.85 torr) pressure values were obtained by studying naturally incubated eggs of the Andean gull (Larus serranus) at 4650 m. Sea-level chicken (Gallus gallus) air cell pressure values of O2 (102.3 +/- 2.7 torr) and of CO2 (43.3 +/- 1.3 torr) were obtained from the literature for comparison. Both these values were similar to those found in the alveolar air of humans at sea level (O2: 104.4 +/- 0.4 torr, CO2: 40.1 +/- 0.25 torr) and at high altitude (4540 m) (O2: 50.5 +/- 0.53 torr, CO2: 29.1 +/- 0.37 torr). Despite very large evolutionary changes in morphology and physiology of the respiratory organs, the head pressure of O2 that oxygenates the blood keeps a constant value in the pre-pipping avian embryo and in the alveolar air of adult mammals. This constancy holds valid at high altitude.