Objective. To define the mechanism of formation of orthostatic hypotension in patients with traumatic tetraplegia. Material and methods. The study included 64 patients with chronic (more 6 month) and subacute (less 6 month) tetraplegia; the period of observation was about 30 days after admission to the rehabilitation center. Changes in the state of the autonomic nervous systems (heart rate variability at rest and head-up tilt test, sinus arrhythmia) were studied. Results. In the first 6 months, all patients, and after 6—12 months, one third of the patients experienced orthostatic hypotension, which was accompanied by lower values of the normalized sympatovagal index (LFn/HFn) 0.35 (0.260; 0.650) and expiratory-inspiratory coefficient (RRmax/RRmin) 1.09 (1.040; 1200). An increase in expiratory-inspiratory coefficient as a result of physical rehabilitation up to 1.16 (1.120; 1.24) was noted only in patients less than 6 months after injury. Conclusion. Both sympathetic and parasympathetic parts of the autonomic nervous system are involved in the formation of orthostatic hypotension syndrome in patients with spinal tetraplegia.