1. Seasonal changes in the weights of fat body, gonads and liver, liver-glycogen content, and blood sugar have been followed by observations on monthly samples of Rana temporaria which had been kept in captivity for 3 days without food.
2. Three periods during which the blood sugar was high (from 54 to 62 mg./100 ml. of blood) were observed during the year: at the spawning season in March, in June and July, and in November.
3. The hyperglycaemia of June and July was accompanied by the rapid development of the fat body. This phase was succeeded by that of gonad regeneration, during which there was no further storage of fat or liver glycogen, and the blood sugar was low (about 40 mg./100 ml. of blood).
4. When gonad regeneration was nearing completion glycogen storage in the liver became the dominant feature, and the blood sugar tended to rise again.
5. A sex difference in blood sugar was observed at certain periods. From June to September the female blood sugar was higher than that of the male, but from September to January the relationship was reversed. This may be correlated with the differing rates of gonad regeneration in the two sexes.
6. During the winter months, when the food intake was reduced, the frogs utilized some of their stored fat and glycogen and the blood sugar was low (about 40 mg./100 ml.). The hyperglycaemia found at the spawning season was accompanied by the rapid depletion of the fat and glycogen stores.
7. In April and May, after the breeding season, when the frogs were feeding actively, the blood sugar was low and apparently neither fat nor glycogen was being accumulated. It is suggested that this was a period of tissue repair and possibly of growth.