1. A description is given of the hour-to-hour variation in the liver glycogen content in adult male mice, and it is shown that the concentration is highest while the animals are asleep and lowest while they are awake.

2. A similar cycle is also described in the glycogen content of the skin. Histologically it is shown that a high proportion of the skin glycogen lies in the cytoplasm of the epidermal cells, and that during sleep both the epidermal glycogen content and the epidermal mitotis rate increase considerably. The skin glycogen content and the epidermal mitotic activity also show a marked increase after a subcutaneous injection of 20 mg. starch, while they are both abnormally depressed after two injections of 1/50 unit insulin.

3. These results, together with others previously reported, are in agreement with the theory that at the onset of sleep glucose is deposited from the blood into the tissues where it appears in the form of glycogen. Since it is known that glucose, or glycogen, is a critical substance affecting mitotic activity in the adult mouse, it is logical to find that an increase in the epidermal glycogen content is accompanied by a greatly increased mitosis rate. On waking, the reverse process takes place, glycogen being withdrawn as glucose into the blood and mitotic activity falling to a low level.

Sorby Fellow of the Royal Society of London.