1. The natriuretic activity of an extract from the kidneys was studied in control and nephrotic rats following acute saline expansion.
2. The natriuretic effect of a known natriuretic material prepared from human urine was evaluated in control and nephrotic rats.
3. After actute saline expansion, natriuretic activity was obtained only from the fractions extracted from the kidneys of the control rats.
4. Injection of natriuretic material produced a equal natriuretic response in both control and nephrotic rats.