Respiratory movements, irregular and periodic before, become regular and continuous soon after the chick begins to breathe air about a day before it hatches. The respiratory rate of unhatched air-breathing chicks increases as the time of hatching approaches and the allantois dries. Rapid respiration at hatching is an expression of anoxemia and CO2 accumulation occurring normally. Nitrogen administration served to increase the respiratory rate unless it had reached a peak during the anoxemia of hatching. CO2 in excess O increased the rate of breathing in some but had no effect on depressed respiration in other chicks at hatching. 6% or more CO2 slowed respiration.