Recent evidence from chemical analysis of tissue electrolyte and water composition has shown that body Na⁺ content in experimental animals is not constant, does not always readily equilibrate with water, and cannot be exclusively controlled by the renal blood purification process. Instead, large amounts of Na⁺ are stored in the skin and in skeletal muscle. Quantitative non-invasive detection of Na⁺ reservoirs with sodium magnetic resonance imaging (²³NaMRI) suggests that this mysterious Na⁺ storage is not only an animal research curiosity but also exists in humans. In clinical studies, tissue Na⁺ storage is closely associated with essential hypertension. In animal experiments, modulation of reservoir tissue Na⁺ content leads to predictable blood pressure changes. The available evidence thus suggests that the patho(?)-physiological process of Na⁺ storage might be of relevance for human health and disease.