Weightlessness results, in Man, in a redistribution of organic fluids from the lower half of the body to the thoraco-cephalic areas. This fluid transfer is responsible, in the course of the flight, for a series of subjective and objective disorders which the astronauts will experience. These disorders are related to venous stasis in the thoraco-cephalic area. Moreover, upon return of any space mission, regardless of its length, the astronauts will experience a disadaptation of their cardiovascular system indicated by hypotension with the possible occurrence of a presyncopal state, or even a syncope and a lowered stress capability. The haemodynamic consequences of this fluid transfer, from central and peripheral standpoint, and the mechanisms implicated in the cardiovascular deconditioning syndrome, are presented and discussed.