Abstract: A breathing regulator suitable for regulating aircrew breathing demand for oxygen-enriched air supplied as breathing gas by an aircraft on-board oxygen generating system includes a valve responsive to pneumatic signals from an aircraft mounted, G sensitive valve. The valve throttles passage of gas from a breathing-pressure control chamber to ambient atmosphere so that in high or rapidly increasing G load conditions the pressure in the breathing-pressure control chamber is increased and a positive delivery pressure is provided at the regulator outlet.