1. 1. The effect of fasting on respiratory quotient (RQ), metabolic rate, blood glucose, liver glycogen, carcass lipids, interscapular brown adipose tissue (IBAT), and body temperature was investi- gated in Microtus pennsylvanicus. 2. 2. The utilization of carbohydrates during fasting leads to a severe hypoglycemia within 6 hr. 3. 3. The hypoglycemia does not seem to stem from the inability to mobilize glycogen or fat reserves. 4. 4. The hypoglycemic state may be responsible for the decreased SMR and body temperature. 5. 5. The predominant use of carbohydrates may stem from a high metabolic rate coupled with a low calorie diet which forces the voles to feed frequently. © 1988.