(57) A breathable gas delivery regulator (10) has a demand valve (15) which will open easily to substantially consistent efforts of a user throughout the pressure range of supply gas from a molecular sieve oxygen generator (not shown). The demand valve (15) is balanced by supply gas pressure and is operably connected, such as by a mechanical lever (29), with a pressure-responsive wall (27) of a demand-pressure sensing chamber (12) which is in fluid communication with a gas delivery outlet (17). Movements of the pressure-responsive wall (27) cause the demand valve (15) to move towards opening or closing and thereby regulate the flow of supply gas from a supply gas inlet (16) to the gas delivery outlet (17).