1. Serum was collected from normal rats and from rats volume-expanded with isotonic sodium chloride solution.
2. The serum was fractionated by gel filtration on Sephadex G-25 and each fraction was tested for inhibitory activity against sodium—potassium-activated adenosine triphosphatase prepared from rat kidney homogenate.
3. A single low-molecular-weight fraction, eluting after the salts and after exogenously added lysine-vasopressin, had significantly greater enzyme inhibitory activity when obtained from serum of volume-expanded animals than from control serum.
4. As this fraction has been shown in previous independent studies to contain a natriuretic factor, it may be concluded that one property of this factor is the ability to inhibit sodium—potassium-activated adenosine triphosphatase.