an improved breathing method and apparatus for enabling a user , while at a first altitude , e . g . sea level , to experience higher altitude conditions . the apparatus includes an inspiratory tube and an expiratory tube and means for mixing a portion of oxygen - reduced expired air with ambient air to supply air for inspiration . the first ends of the inspiratory and expiratory tubes are coupled to the users airway through a user breathing means . the remote end of the expiratory tube is coupled through a co 2 absorber canister to an air storage reservoir . the remote end of the inspiratory tube is also coupled through a co 2 absorber canister to the reservoir for pulling oxygen - reduced air therefrom . the user breathing means includes a selectively dimensioned orifice to the environment so that on expiration , a predetermined portion of the user &# 39 ; s expired air is exhausted to the environment while the remainder passes through co 2 absorber material into the reservoir . on inspiration , air stored in the reservoir is pulled through co 2 absorber material and combined with ambient air pulled into the breathing means through the orifice . different size orifices can be utilized to simulate different altitudes .