1. The relationship of changes in plasma renin and plasma aldosterone concentration to electrolyte balance was studied during total fasting and during sodium deprivation followed by total fasting.
2. During simple sodium deprivation obese subjects lost significantly more sodium than lean subjects, but the rise in plasma renin concentration (PRC) was similar in the two groups.
3. During total fasting there was a failure of PRC to increase in the expected manner despite a marked negative sodium balance. In the early stages of the fast, PRC decreased in nine of eleven subjects while subsequently it increased in all subjects.
4. During sodium deprivation PRC and plasma aldosterone concentration (PAC) rose in all subjects studied, but in the first few days of a period of total fasting immediately following, there was a fall in PRC in eight of ten subjects studied, while PAC continued to rise in five of six subjects in whom it was measured.
5. These results are discussed in relation to the concept of the renin-angiotensin system as a regulator of aldosterone secretion.