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SUMMARY The effects of progesterone administration on the weight and composition of the body have been studied in rats. Female rats injected with 5 mg. progesterone/day initially gained weight at an average rate of 2 g./day, compared with 0·4 g./day for controls. When treatment was continued for a month or more their weight stabilized at 40–50 g. above the control level. The bodies of the progesterone-treated rats contained increased amounts of water, fat and solids other than fat. These effects were smoothly related to the dose of progesterone. In terms of percentage composition, fat increased at the expense of the other two constituents. The composition of the fat-free solids did not change, but the proportion of water in the fat-free body increased. About a tenth of the gain of live weight was accounted for by an increase in the contents of the alimentary tract. The composition of the rest was equivalent, typically, to 43% lean tissue, 26% water additional to that in the lean tissue, and 31% fat. Male rats treated with progesterone showed no changes other than a small gain of water. It seems likely that in females progesterone reproduces the changes in body composition which occur in pregnancy. The gain of lean tissue seems to reflect increased growth it, and the accumulation of fat, may both be consequences of the production of a positive energy balance.

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Excessive food intake and obesity was induced in one member of parabiotic pairs by electrical stimulation (three 30-min sessions/day for 2 wk) of the lateral hypothalamus (LH). The nonstimulated partners reduced spontaneous food intake the fatter the stimulated animals became. This reduced food intake resulted in a decreased body weight, fat content, and fat-free solid body mass. The decrease of food intake was not due to changed social behavior of the obese partner. It must be attributed to transmission of a humoral satiety factor. The very first stimulation of the LH in the stimulated partners resulted in a large increase in blood glucose and glucagon level without much change in the insulin level. These changes in blood parameters were probably due to strong sympathetic arousal. In the nonstimulated animals there were practically no changes in these parameters. One week of fattening resulted in increased basal glucose and insulin levels in the stimulated animals and decreased glucose levels in the nonstimulated partners, in which the basal insulin levels remained nearly normal. Basal glucagon levels were the same in both partners and did not differ from the prefattening situation. At that time during stimulation the obese animals showed a large increase in glucose and glucagon levels and a decrease in insulin level. On the other hand the nonstimulated animals showed a slow gradual increase in glucose and insulin level due to transmission from their fat partners because of the large gradient in these substances between the animals. These phenomena were still more pronounced after 2 wk of fattening. It is tentatively concluded that the humoral satiety factor is neither circulating insulin nor glucagon nor one of the major circulating nutrients.