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The tolerance of neonatal rabbits to asphyxiation in 95% N₂ + 5% CO₂ was studied under conditions of controlled body temperature, and the enzymic activities of succinoxidase and lactic dehydrogenase were quantitatively determined in the heart and brain of untreated litter-mates.

1. In new-born rabbits tolerance to asphyxia increases progressively as body temperatures are reduced until at 15° C it is four times that at 39° C. At colonic temperatures below 15° C, tolerance to asphyxia decreases rapidly.

2. All new-born rabbits having colonic temperatures of 15° C recover spontaneously from asphyxia two to four times as long as that which is lethal for their warm litter-mates. Recoveries from asphyxia five and six times as long occur in 80%.

3. At 39° C colonic temperature, the tolerance of new-born rabbits to asphyxia decreases over ninefold from birth to young adulthood.

4. In all tissues studied succinoxidase activity increases during the post-natal period. In the heart and frontal lobe, lactic dehydrogenase activity tends to parallel that of succinoxidase and is lower in the new-born than one might expect in view of the tolerance to asphyxia at this time.

5. In the medulla oblongata an increase in succinoxidase activity occurs without a parallel increase in lactic dehydrogenase activity. This may reflect an increased dependence on aerobic pathways during post-natal development, with no parallel increase in the capacity for anaerobic glycolysis. Such changes may explain in part why the sensitivity of the medullary respiratory centre complex to oxygen lack is so much greater in the adult than in the new-born.