Two unusually large male rats maximum weights 830 and 766 grams fasted 57 and 38 days, respectively, when one was killed for autopsy and the other was refed. In neither instance did the animal appear moribund. At the end of the fasts the body weights were 46 and 55 per cent of the initial weights 822 and 730 grams. With both rats the daily loss in weight remained relatively constant, after an initial pronounced decrease, until toward the end of the fast when the drafts upon body weight were heavier. The autopsy of the rat that fasted 57 days showed it still had a liberal supply of fat. In a comparison of the basal metabolism of the very large rats with smaller rats, it was found that the metabolism was lower per unit of weight and per unit of surface area, the larger the rat. Male rats weighing from 307 to 396 grams produced on the average 92 calories per kilogram of body weight and 708 calories per square meter of body surface per 24 hours. Male rats weighing from 407 to 562 grams produced 77 calories and 664 calories, respectively. The two unusually large rats produced 59 and 63 calories per unit of weight and 603 and 621 calories per unit of surface area. The rectal temperatures of the very large rats 37.4° were similar to the body temperatures of the medium-sized rats 37.6° and the large-sized rats 37.5°C.. During the prolonged fasts the total oxygen consumption decreased until about the 20th day of the fasts, but thereafter remained relatively constant, although the rats were losing weight rapidly. The heat production per kilogram of body weight and per square meter of body surface decreased markedly during the first few days, but toward the end of the fasts definitely increased. This is in striking contrast to the reaction shown by fasting female rats, whose heat production per unit of weight was essentially constant after the first one or two days, and per unit of surface area decreased as the fast progressed. With the two very large rats there was no increase in cell temperature or increase in activity to account for the increased metabolism toward the end of the fasts. Indeed, the rectal temperatures during the prolonged fasts were not different from those of rats under normal conditions. These findings suggest that there was an increased protein disintegration toward the end of the fasts, although the nitrogen in the excreta was not determined. With one of the very large rats minimum values of 377 and 381 calories per square meter of body surface per 24 hours were recorded on the 29th and the 43d days of fasting. This is probably the lowest heat production that has been noted with warm-blooded animals having a rectal temperature of essentially 37°C.