1. 1. The ambient temperature of embryos of pipped eggs and hatchlings was reduced from 38 to 28°C for a period of 1 hr. 2. 2. The weight-specific rate of oxygen consumption ( V ̇ O 2 ) of embryos and hatchlings at 38°C was le than the predicted value for adults of the same weight. 3. 3. The Q 10 value for mean weight-specific V ̇ O 2 was 2.08 for pipped eggs, 0.98 for 4-hr-old and 0.67 for 1-day-old chicks. 4. 4. The Q 10 values for V ̇ O 2 of individual embryos fell in the range 1.69–2.42, but the Q 10 values for ventilatory frequency fell between 1.22 and 4.65. There was no correlation between the Q 10 values for V ̇ O 2 and ventilatory frequency for the same embryos. 5. 5. When ambient temperature was returned from 28 to 38°C V ̇ O 2 and ventilatory frequency returned to pre-cooling values within a period of 1 hr. 6. 6.The results are discussed in relation to the factors which influence gaseous exchange during the perinatal period.