The objective of this study was to valid a ground-based model for microgravity and zero gravity and to study the effects of simulated condition salivary cortisol. Because 6° head-down tilt (HDT)and bed rest position (HB) an established method to mimic low gravity and zero gravity on earth, the aim of the present study was to determine the effects of 20-day HDT &HB on salivary cortisol levels in 10 healthy male volunteers. The cortisol was measured in saliva. During HDT and HB, all volunteers the diurnal rhythm of cortisol  secretion was significantly increased in simulated conditions. Thus 6° HDT & HB appears to be a valid model to induce psychic stress due to increased levels of cortisol, changes that might also be encountered by astronauts and marsonaunts during long-duration spaceflights.

Key words: Head-down tilt, Bed rest position, cortisol, saliva.