Oxygen consumption of Rhea (Rhea americana) and Emu (Dromiceius novaehol- landiae) eggs increases exponentially during the first 70% of incubation and reaches a maximum about three-quarters of the way through incubation. Rate of 02 consumption then declines to about 75% of the peak value, increasing again just prior to pipping. We suggest the decline in rate of 02 consumption is due to a decline in growth rate, and that growth of the embryo of ratites is essentially complete at the time of the peak in 02 consumption. Completion of growth prior to the normal end of incubation may permit ratite eggs of different ages to synchronize hatching within a clutch. Rates of 02 consumption just prior to the initiation of pulmonary respira- tion are 104 + 7 cm3 h-1 in Rhea eggs and 75 + 7 cm3 h-1 in Emu eggs. Calculated and measured air-cell gas tensions at this stage of incubation vary systematically with egg size between species of birds. Large eggs have higher air-cell 02 tensions and lower air-cell CO2 tensions than do small eggs. Water vapor conductance of Emu eggs is 46.2 + 9.6 mg- day-' torr-1, much lower than predicted on the basis of egg size and incubation period.