ABSTRACT. Many newborn mammals decrease metabolism and body temperature (Tb) during acute hypoxia. We asked what effects warming of the hypoxic newborn would have on these variables. In unanesthetized newborn cats and dogs, we measured the breathing pattern, C 0 2 production, Tb, and ambient temperature during normoxia at an ambient temperature of 28"C, 30 min of hypoxia (10% 02), and an additional 30 min of hypoxia plus warming. During hypoxia, Tb and COz production decreased in both species, whereas the absolute value of ventilation did not change in kittens and increased in puppies. During hypoxia plus warming, Tb was gradually returned to the normoxic value by increasing ambient temperature by 3 to 4.5OC. This increase did not modify C 0 2 production in either species, and it increased minute ventilation in kittens. We conclude that during hypoxia 1 ) warming the newborn can increase Tb, but not metabolism, to the normoxic value; 2) the decrease in Tb is not a causative prerequisite of the hypoxia-hypometabolism; and 3) an artificial increase in Tb to the normoxic value can stimulate minute ventilation, probably because it is perceived as an hyperthermic stimulus. In many newborn mammals, including infants, metabolic rate and Tb decrease during acute hypoxia (1-7). The mechanisms responsible for these phenomena are not understood, nor is it clear whether the drop in Tb and the metabolic adaptation are linked by a cause-effect relationship. In newborn cats, hypoxia would not lower oxygen consumption if the animals were maintained at about 34"C, which is their normoxic thermoneutral value (8), whereas both Tb and metabolism decrease during hypoxia at 28 to 30°C (2, 5, 6), a range of Ta that is similar to or slightly higher than that experienced in the nest (9). In newborn rats, metabolic rate decreases during hypoxia even at Ta values equivalent to the lower end of their normoxic thermoneutral range (3, 10). One hypothesis, therefore, could be that, in a newborn, even a small hypothermic stress favors the hypometabolic response during hypoxia. If this interpretation were correct, it would follow that in a hypoxic newborn both T b and metabolism could be returned to the normoxic values by means of external body warming.An alternative possibility could be that during hypoxia T b decreases as the result of the hypometabolism and reduced heat production and adjusts to what would represent the normal value for that metabolic conditions, similarly to the mechanism of T b resetting of hibernators (1 1). In such a case, external warming of the hypoxic newborn would act as an hyperthermic challenge, which could raise Tb but not necessarily increase metabolic rate.In these experiments, we attempted to gain some insights into the relationship between body temperature and metabolic rate of the hypoxic newborn by simulating a condition of acute hypoxia in newborn cats and dogs at first maintained at the average Ta of their nest. As expected, we observed a drop in Tb and Vco2. With the animals under continuous hypoxia, we t...