Newborn rats exposed at 37 C to pure nitrogen continue to breathe slowly for 22 minutes. As they grow older the time to the last breath diminishes, and the pattern of breath frequencies changes. At 4 to 11 days of age pretreatment with hypoxia for a few hours prolongs the time to last breath for 40 to 75%. To that prolongation (physiological adaptation) may be added the smaller effect of immediate glucose injection. Neither prolongation is available in the first two days after birth. The adaptation appears to modify the regulation of anaerobic breathing in the medulla oblongata, which acquires this specific superfunction in response to the pretreatment with hypoxia.