**5. Conclusions**

In conclusion, the results of this study showed that maximal physical exercise, regardless of whether it is performed under normoxia or mild normobaric hypoxia (equivalent of 3000 m altitude) caused a similar increase in BDNF concentration in the blood of well-trained athletes. Despite the fact that BDNF has been known to possess a protective effect on the brain, an elevated BDNF level did not protect our participants from cognitive impairment due to acute exposure to hypoxia, because indices of this variable, i.e., CRT and NCR, were worse in hypoxic conditions. All examined potential circulating factors that are known to affect BDNF expression/production (C, nitrite, ET-1, catecholamines, LA) most likely did not affect psychomotor functions in our experimental paradigm.

**Author Contributions:** Conceptualization Z.P., M.C. (Miłosz Czuba), and J.L.; methodology, Z.P., M.C. (Miłosz Czuba), and J.L.; formal analysis, K.P. and M.C. (Miłosz Czuba); investigation, Z.P., M.C. (Małgorzata Chalimoniuk) and M.C. (Miłosz Czuba); data curation, M.C. (Małgorzata Chalimoniuk) and K.P.; writing—original draft preparation, Z.P., M.C. (Małgorzata Chalimoniuk), K.P., M.C. (Miłosz Czuba), J.L.; writing—review and editing, Z.P., M.C. (Miłosz Czuba) and J.L.; visualization, Z.P. and K.P.; supervision, M.C. (Małgorzata Chalimoniuk) and J.L.; funding acquisition, J.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was supported by the grant No. 2013/09/B/NZ7/00726 from the National Science Centre of Poland.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
