A concept called model individualization is presented. It is used to modify computer based models to reproduce observed individual behavior. During D2-, MIR97- and Neurolab-missions tilt-table and LBNP-experiments were carried out. Physiological data describing the cardiovascular reactions of the astronauts were recorded. The appropriateness of the rheoretical principles is demonstrated with MIR97 tilt-table experiments. Finally the resulting individualized model is investigated to propose hypotheses on probable alterations in the cardiovascular system induced by microgravity.