The work is devoted to physiological aspects of application hyperbaric oxygenation (HBO) in medical practice. Is investigated vegetative reactivity to HBO at a multiple sclerosis and at the liquidators of accident on Chernobyl atomic electrical server with dyscirculatory encephalopathy. For study vegetative reactivity heart rate variability was used. The comparative data on a modification high-frequency component of the spectral analysis of an heart rhythm immediately in time HBO sessions. The existence group variability to hyperoxia irrespective of disease is shown. Is concluded about priority of a functional principle of a choice of a HBO regime.