The effects of a natriuretic factor contained in extracts of the atrial myocardium on an isolated renal Na+, K+-ATPase enzyme system were evaluated. Ultrafiltrates (molecular weight < 30, 000) of boiled extract of rat atria and ventricles were prepared. Infusion of 100 μl of the atrial ultrafiltrate into bioassay rats resulted in a prompt, short-lived natriuresis and diuresis. However, addition of 100 μl of the atrial ultrafiltrate to 900 μl of a suspension containing Na+. K+-ATPase had no significant effect on enzymatic activity. Similarly, ultrafiltrates of ventricular extract also had no significant effect on Na+. K+- ATPase activity. These results indicate that the atrial natriuretic factor does not alter renal tubular sodium reabsorption by directly inhibiting the Na+, K+-ATPase enzyme system. © 1983 S. Karger AG, Basel.