1. The electrolyte and water composition of urine and ileal fluid was studied in six patients with ileostomies during dietary sodium deprivation and at the same time measurements were made of plasma renin activity, plasma aldosterone concentration, renal aldosterone excretion and plasma arginine vasopressin concentration.
2. In each subject there was a reduction in renal sodium excretion within 24 h of sodium deprivation. Significant rises in plasma renin activity, plasma aldosterone concentration and renal aldosterone excretion occurred by day 2 of sodium deprivation.
3. There was only modest conservation of sodium by the ileum during sodium deprivation, but ileal potassium concentration rose progressively. There were significant associations between ileal-fluid potassium concentration and plasma renin activity or renal aldosterone excretion.
4. Under the conditions of the experiment a diuresis was observed in each subject during dietary sodium deprivation. This did not appear to depend on changes in plasma arginine vasopressin concentration.
