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 (carbon dioxide) CO.sub.2 absorber canister to an air storage reservoir. The remote end of the inspiratory tube is also coupled through a CO.sub.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's expired air is exhausted to the environment while the remainder passes through CO.sub.2 absorber material into the reservoir. On inspiration, air stored in the reservoir is pulled through CO.sub.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.