We have recently shown that 'normal' amounts of sodium intake in space as well as high NaCl intake on Earth may lead to sodium retention without concomitant fluid retention. When we further examined the impact of the onset level of NaCl intake we found that increasing NaCl ingestion from a low NaCl intake (0.7 mEq/kg body weight (bw)/d) to an average NaCl intake level (2.8 mEq/kg bw/d) caused osmotically active sodium retention. However, a further increase in NaCl intake from average normal NaCl to high levels (7.7 mEq/kg body weight/d) led to sodium retention without fluid retention and without potassium exchange. In another study we then used 6U head-down tilt bed rest (HDBR), a model to simulate some of the physiological changes in microgravity and applied high NaCl intake. HDBR per se led to diuresis in both groups. Hence, in the high NaCl-HDBR phase sodium was retained similarly to the ambulatory conditions. However, in this case about 80% of the retained sodium was accompanied by fluid retention and potassium loss. We conclude from these results that in space as well as in ambulatory conditions sodium was retained osmotically inactive. But, when in HDBR almost all sodium was retained in an osmotically neutral form.