1. Recent studies in our laboratory suggest that Na+,K+-dependent-ATPase and Na+—K+ pump activities are reduced in cardiovascular muscle of animals with experimental low-renin hypertension. 2. We here show that these abnormalities are associated with a heat-stable ouabain-like agent in the plasma. 3. In the volume-expanded rat, an anteroventral third ventricle (AV3V) lesion decreases the level of the ouabain-like humoral factor and increases vascular Na+—K+ pump activity. These findings suggest that the humoral ouabain-like factor originates in or is influenced by the AV3V area of the brain and may help to explain why the AV3V lesion prevents or reverses one-kidney renal and one-kidney, DOCA—salt hypertension in rats.