Breathing assistance apparatus capable of operating in alternating inhalation and expiratory phases, the apparatus comprising: a pressurized respiratory gas source, control means for regulating the operation of the pressurized respiratory gas source capable of transmitting a reference value of a gas related parameter to the pressurized respiratory gas source, the control means including selection means capable of selecting a pressure parameter or a flow rate parameter to define the reference value of the gas related parameter, an inhalation duct to supply gas from the pressurized respiratory gas source to a patient, an expiratory duct for expiratory gas of the patient, an inhalation valve disposed on the inhalation duct, the inhalation valve comprising means to allow the gas from the gas source to pass to make possible proportional operation, the inhalation valve being controlled by means which are distinct from the pressurized respiratory gas source, an expiratory valve on the expiratory duct to help establish a positive expiratory pressure, a pressure sensor and a flow rate sensor associated with the inhalation duct, and an automatic control unit for controlling the selection means, the automatic control unit being connected to the pressure and flow rate sensors (111, 112) to form a direct closed regulation circuit for selecting a reference value parameter, and associated to a program allowing a selection to be made in real time from a pressure signal or flow rate signal, so that association of a direct closed regulation loop for the selection of a particular reference value parameter with the inhalation valve permitting proportional operation allows real time control of barometric and volumetric operating modes of the breathing assistance apparatus, between or during the inhalation and expiratory phases. a pressurized respiratory gas source, control means for regulating the operation of the pressurized respiratory gas source capable of transmitting a reference value of a gas related parameter to the pressurized respiratory gas source, the control means including selection means capable of selecting a pressure parameter or a flow rate parameter to define the reference value of the gas related parameter, an inhalation duct to supply gas from the pressurized respiratory gas source to a patient, an expiratory duct for expiratory gas of the patient, an inhalation valve disposed on the inhalation duct, the inhalation valve comprising means to allow the gas from the gas source to pass to make possible proportional operation, the inhalation valve being controlled by means which are distinct from the pressurized respiratory gas source, an expiratory valve on the expiratory duct to help establish a positive expiratory pressure, a pressure sensor and a flow rate sensor associated with the inhalation duct, and an automatic control unit for controlling the selection means, the automatic control unit being connected to the pressure and flow rate sensors (111, 112) to form a direct closed regulation circuit for selecting a reference value parameter, and associated to a program allowing a selection to be made in real time from a pressure signal or flow rate signal, so that association of a direct closed regulation loop for the selection of a particular reference value parameter with the inhalation valve permitting proportional operation allows real time control of barometric and volumetric operating modes of the breathing assistance apparatus, between or during the inhalation and expiratory phases.