This paper is part of a series titled “AltitudeOmics” that together represent a group of studies that explore the basic mechanisms controlling human acclimatization to hypoxia. This study aims at analyzing the baroreflex sensitivity (BRS) at sea level, in acute and chronic hypoxia and test whether hyperoxia reverts the hypoxic effects. Twenty-one young, healthy subjects underwent experimental trials near sea level, and two times at high altitude. BRS was calculated using the sequence method, spectral method, transfer function and the standard deviation method. Results showed a decrease in BRS with hypoxic exposures. Hyperoxia reverted the hypoxic effects in acute hypoxia only. In chronic hypoxia, BRS reset to lower values but behaved comparably to normoxia.