Experiments were carried out on healthy volunteers exposed to a 7-day water immersion. The subjects were kept dry by use of a water proof, highly elastic cloth. During the experiment different cardiovascular, fluid-electrolyte and biochemical parameters were recorded. During the 7-day immersion, physiological parameters changed on a phasic basis. An acute period of adaptation began within the first minutes of immersion and ended by the third day. Later some of the parameters showed relative normalization. The experimental results suggest that the hypophyseal-adrenal system and renal function are closely coupled regulatory mechanisms for adaptive reactions of the cardiovascular system and for fluid-electrolyte metabolism during acute adaptation of the human body to reduced gravity.