Abstract: Oxygen is discharged from a source of supply into the cabin of an airplane under the control of an oxygen sensing device to maintain the supply of oxygen in each volume of air at a comfortable level but without increasing the pressure of the atmosphere in the plane above the pressure of the atmosphere at the altitude at which the plane is flying. Expressed in other terms, as the plane ascends to altitudes above a preset level, the partial pressure of oxygen is maintained at a preset comfortable level while the absolute pressure in the cabin decreases with the decrease in pressure of the atmosphere outside the plane and remains at a level above the preset height. This eliminates pressurizing the cabin or the need for breathing masks.