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Summary: The steady-state concept of Na+ homeostasis, based on short-term investigations of responses to high salt intake, maintains that dietary Na+ is rapidly eliminated into urine, thereby achieving constant total-body Na+ and water content. We introduced the reverse experimental approach by fixing salt intake of men participating in space flight simulations at 12 g, 9 g, and 6 g/day for months and tested for the predicted constancy in urinary excretion and total-body Na+ content. At constant salt intake, daily Na+ excretion exhibited aldosterone-dependent, weekly (circaseptan) rhythms, resulting in periodic Na+ storage. Changes in total-body Na+ (±200–400 mmol) exhibited longer infradian rhythm periods (about monthly and longer period lengths) without parallel changes in body weight and extracellular water and were directly related to urinary aldosterone excretion and inversely to urinary cortisol, suggesting rhythmic hormonal control. Our findings define rhythmic Na+ excretory and retention patterns independent of blood pressure or body water, which occur independent of salt intake. [Copyright &y& Elsevier]