In newborn and adult rats, hypoxia decreases metabolic rate, especially at low ambient temperatures (Ta). We examined whether a similar effect can occur during hypercapnia. 2. We measured metabolism (oxygen consumption, Vo2, open flow-through method), and expiratory ventilation (VE; barometric method (adults), airflow plethysmograph (newborns)) in air and 2% or 5% CO2 in normoxia. 3. In adults, Vo2 was higher at Ta = 10 degrees C than 25 degrees C. At each Ta, CO2 breathing did not change Vo2, but increased VE, less at 10 degrees C (up to +100%) than at 25 degrees C (+161%). Blood pressure was maintained at both values of Ta and CO2, while pulse rate and body temperature were decreased in 5% CO2 at 10 degrees C. 4. In newborns, the metabolic response to lowering Ta (from 40 to 20 degrees C) much depended on behavioural responses, being larger in groups of two or four pups than in individual animals. In no case did CO2 influence the response. VE increased during 5% CO2 exposure, more so at Ta = 33 percent C (+69%) than at 25 degrees C (+49%). 5. In both adults and newborns, hypoxia (10% O2) always decreased metabolic rate. 6. We conclude that hypercapnia has no appreciable effects on metabolic rate in rats (both newborns and adults) even at low Ta, a result quite different from the hypometabolic response to hypoxia.
