1. Embryos and hatchlings of the duck and pigeon were exposed to a lowered temperature for 6 hr. The oxygen consumption (MO2) was measured before and after cooling and the ratio of the two was compared with that predicted for a temperature coefficient of 2 (Arrhenius value). 2. Late prenatal ducks kept the MO2 above the Arrhenius value at 28 degrees C, while the MO2 of pigeon hatchlings became the same as the Arrhenius value even at 32 degrees C. 3. Thus, incipient homeothermic ability appears in the duck during prenatal development, but it is not evident in the pigeon even after emergence from the shell. The precocial chicken and semi-precocial noddy previously studied are intermediate in their metabolic response between the duck and the pigeon.