1. The blood volume of 18 rats was expanded with blood with which they were equilibrated. In another 18 rats undergoing the same experimental protocol, the blood volume was not expanded.

2. In some rats the urine was reinfused.

3. A 1 ml portion of plasma obtained either 1 h after the blood volume was expanded, or at an equivalent time from the control rats, was injected intravenously into an assay rat.

4. The injection of plasma produced a gradual and significant rise in sodium excretion in the assay rat when it was obtained from an animal with an expanded blood volume, the urine of which was being reinfused.

5. When the increase in blood volume was sustained by a preceding bilateral nephrectomy, instead of urine reinfusion, the injection of 1 ml of plasma into an assay rat did not cause a rise in urinary sodium excretion.

6. It is concluded that the natriuresis involved in the assay rat was caused by a natriuretic substance in the 1 ml of plasma obtained from the blood-volume-expanded urine-reinfused rat.

7. The experiments suggest that the natriuretic substance is excreted in the urine and may be produced in the kidney.

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