The rate at which uric acid is turned out of the body is very different at different times of the day, even when the food contains no ready-made purine derivatives. It is higher during the early hours of the day than at anytime, and it is considerably lower at night. The reason for this, as was pointed out by one of us, is not likely to be that the excretory functions are depressed at night, since these functions, to judge from the total nitrogen of the urine, are more active during the first hours of sleep than at any time in the 24 hours. And since, when the diet is confined to bread, butter, and milk, the uric acid must be derived from the body substance and not from the food, it seems probable that there is some function of the body which is in abeyance during sleep and is, to a considerable degree, responsible for the output of uric acid; some function, that is to say, which is effected by chemical reactions involving the production of uric acid, and possibly in some measure creatinine. If it is possible to identify this function, the activity of which can, on a suitable diet, be measured by the amount of the uric acid excreted, it may be possible to give a clearer account of the processes by which, at the onset of fever, the temperature of the body can be sent up independently of any voluntary muscular activity; for while the temperature is rising, the output of uric acid may be four times as great as it otherwise would be. Similarly, in the study of other pathological conditions in which uric acid plays a part, it must be of importance to be able to point to the kind of activity which is accompanied by increased uric acid production.