‘Dry’ immersion is a commonly accepted simulation of microgravity conditions (Tomilovskaya et al., 2019). Investigation of disorders in hemodynamic regulation , caused by immersion, can furnish insights into the nature of orthostatic intolerance observed in astronauts upon return to earth. 8 healthy young men were in «dry» immersion for 21 days. The day before, on 7th, 14th and 19th day of immersion, as well as on 1st and 5th day after, all participants performed head-up tilt test (TT): 15 min in supine position (0°) then 15 min ortostasis (60°). During the test ECG (PneumoCard, MCS, Russia), arterial pressure, stroke volume (Finometer, Finapres Medical System, The Netherlands) and respiration rate (nasal thermistor sensor) were recorded. Index of phase synchronization (PSI) of beat-to-beat mean arterial pressure (MAP) and heart rate (HR) at frequency range of baroreflex waves (~0.1 Hz) was used as a measure of baroreflex activity (Borovik et al.,2014). Similar measurements during prolonged (up to 40 min) TT were performed on a group of patients with diagnosed vasovagal syncope. Long stay in dry immersion leads to drastically alterations in hemodynamic response to the change of body position (Table 1). In immersion, participants had tachycardia and reduction in SV even at a horizontal position of the body, there was also a more pronounced increase of HR during orthostasis. Slight decrease in pressure during orthostasis can be noted also. Before immersion, a transition from supine position to orthostasis was accompanied by an increase in PSI, in contrary during immersion PSI did not change. All measured parameters returned to initial values at 5th day after immersion. Patients were divided into 2 groups: who did not show any evidence of syncope during orthostasis (TT- group) and who showed signs of pre-fainting (TT+ group). In TT- group (N=13), as well as in control group (N=13), a transition from supine position to orthostasis increased PSI (from 0.18±0.05 to 0.28±0.09*, respectively, p<0.05) in baroreflex frequency range, in contrast to TT+ group (N=11) in which PSI did not change. CONCLUSION: The lack of increase of phase synchronization of MAP and HR at the frequency range of baroreflex waves during TT indicates disorders of baroreflex regulation of hemodynamics and, as a consequence, the inability to maintain blood pressure at the level necessary to support the permissible level of cerebral perfusion.