A prototype for the control of blood gases during extracorporeal circulation (ECC) has been built. It is composed of a CDI300 continuous gas blood analyzer, a programmable gas blender and an IBM PC. The air blender is composed of three mass flow controllers (air, oxygen and total flow rate). The microcomputer commands these controllers in order to obtain the desired mixture of air and oxygen. The system acquires the data sent by the blood gas analyzer every 6 seconds and commands the gas blender in order to maintain the desired arterial dioxide partial pressure (PaCO2) in the arterial line. The first experimental results on sheep show that if blood gas parameters are stable when beginning the closed loop control, then the desired PaCO2 value may be obtained. Otherwise, the value of PaCO2 under control is slightly different but a stable value is obtained after 10 minutes. More experiments must be done in order to establish the real limits of such a system and optimize the gain of the control system.