We examined the effects of varying dietary NaCl on blood pressures of young adult men in two standardized metabolic ward studies: an ambulatory study during which the subjects (n = 9) were sequentially submitted to four different dietary NaCl regimens (0.7, 2.8, 7.7, and 0.7 mEq/kg body weight (bw) /d), and a head-down bed rest (HDBR) study. HDBR is a model to simulate some of the physiological changes induced by microgravity. This study had a randomized balanced cross-over design, with n = 8 and two different NaCl intake levels (0.7 and 7.7 mEq/kg bw/d). Twenty-four hour blood pressure monitoring showed that blood pressures were lower during the high- than during the low-NaCl-intake periods in both studies, ambulatory and HDBR. This was especially remarkable for diastolic pressures during the night period. We further observed that, during HDBR, low dietary NaCl intake induced a non-dipper pattern of diastolic pressure, while high NaCl diet maintained the circadian pattern, even though at a lower level than under ambulatory conditions. The different dietetic NaCl intakes caused changes in extracellular and plasma volumes, and modulated the activity of the RAAS and, likely, of the sympathetic nervous system. All this contributed to the differentiated subjects' blood pressure responses to low and high dietetic NaCl under both conditions, ambulatory and HDBR.