An account of the seasonal changes in the weights of woodchucks, Marmota monax monax, is given for three age classes: (1) young of the year, (2) yearling, and (3) adult. The weight of the retroperitoneal white fat below the diaphragm and to the right of the dorsal aorta was taken as an index of the total amount of fat to illustrate the cyclic deposition and utilization of fat by these animals during the year. Woodchucks attain progressively higher weights each year for the first three years of life at least. They gain and lose weight periodically, gaining weight for about six months, then losing weight for the next six. The mean rate of increase in weight was highest, 16–20 grams per day (April through September), in the young of the year, but diminished to 14.7 grams per day (May through September) in the yearling class and 11.5–13.0 grams per day (April through August in the adult class. Total losses during the period of decline in weight ranged from 20 per cent of the prehibernal weight of young males to 37 per cent of the prehibernal weight of adult males. The rate of loss of weight immediately after hibernation was much higher than the rate during hibernation. Much of the fat stored prior to hibernation was still present in late winter when the animals emerged from hibernation. Adult males emerged from hibernation approximately a month before the females and the yearling males. Fat was utilized as a source of energy during the period of scarcity of food which followed hibernation. Fat was also an important source of energy for the females during the spring breeding season, as they utilized fat reserves during gestation and lactation.