Abstract: A isothermal rebreathing apparatus and method for collecting human breath samples for chemical analysis of the ethyl alcohol content wherein the breath samples are obtained from a subject repetitively exhaling into the inhaling from an enclosed, flexible, variable volume, bag-like collection receptacle which is heated to maintain the breath sample enclosed therein at a prescribed temperature, preferably at or near the subject's body temperature, before extracting the breath sample for subsequent chemical analysis. The collection receptacle may be enclosed by an airtight chamber including heaters therein to warm the air around the collection receptacle in order to regulate the temperature of the breath sample therein. A bellows section in fluid communication with the heating chamber allows for inflation and deflation of the flexible collection receptacle while located in an airtight environment.