The oxygen uptake of small (bantam hen) and large (white leghorn) domestic fowl eggs was measured during incubation in air and pure O2. 2. A 2 hr exposure to pure O2 increased the O2-uptake from the 9th day in the large eggs and from the 15th day in the small eggs. 3. On the last 4 days of the incubation period, both small and large eggs increased their O2-uptake by about 22% when transferred from air to pure O2. 4. When 1 cm2 of the eggshell above the air cell was removed, the O2-uptake of the white leghorn eggs incubated in air increased by 13%.