During the whole course of clinical hypoxyradiotherapy, patients may experience with 25-35 sessions of the sustained hypoxic disciplines. It is quite possible for them to develop some compensation or adaptation to hypoxia. In the present study, analysis of parameters, derived from both hypoxic oxygen tolerance tests before and after the course, showed the evidence of the compensatory mechanism to transient systemic hypoxia induced by breathing hypoxic gas. The conclusion may be different from what is generally believed. Its physiologic basis, development and clinical tactics for counteraction were investigated.