A controlled enhanced-oxygen (hyperoxic) breathing system involving an oxygen source, a control valve system and a breathing interface, is disclosed. The control valve system may include a demand flow valve and a venturi valve arranged to introduce ambient air into an oxygen stream to provide a desired level of oxygen in the resultant hyperoxic air stream. Alternatively, the control valve system may include a nitrogen-removal unit, such as nitrogen scrubber, to increase the oxygen content above that of ambient air. The control valve system may further include a one-way flow valve and may further be joined to a plenum for storing hyperoxic air. The hyperoxic breathing system is typically connected to a breathing mask for use by an individual undergoing exercise or physical training activities.