The maintenance of hemodynamic stability is a functional characteristic of the cerebral circulatory system. This characteristic developed during the evolutionary process. It can be of use as an indicator of the activity of the central processes regulating cerebral blood flow under different stress conditions, particularly, during a long space mission. Rheoencephalographic methods are qualitative and yield information that permits assessment of the changes in the functional status of the cerebral circulatory system. This was confirmed by results obtained during the Salut-4 space mission. The observations permitted us to assess indirectly the status of the cosmonauts.