In the first comparative consideration of the metabolism of avian embryos, Rahn et al. (1974) made several important assumptions and offered a number of interesting predictions. One of their assumptions was that the relation between metabolic rate and incubation age (days incubated) is the same in all avian species as it is in the chicken (Gallus gallus). A second important assumption was that the partial pressures of oxygen and car- bon dioxide in the air cell during the plateau phase are essentially the same in all species, regardless of egg mass and incubation period. This assumption led Rahn et al. to predict that, for eggs of different species but of the same mass (W), "plateau" metabolic rate (rioh) * is inversely related to the length of the incubation period (I) and can be predicted most accurately from the equation: lrh2 = 267 W/I (1) Embryonic metabolism increases with egg mass (Rahn et al. 1974). Therefore, the eggs of the Ostrich (Struthio camelus), which are the largest laid by any living species of bird, offer an important opportunity to test the validity of these assumptions and the accuracy of the prediction. In the present study we measured the rate of oxygen consumption of Ostrich eggs during incubation at 35?C and determined the water vapor conductance of the shell. This permitted us to calculate the air cell gas tensions. Additionally, we determined the values of a number of important physical parameters and the effect of moderate temperature changes on metabolic rate.