[en] OSIRIS, one of the French C.E.A. research reactors located at SACLAY, near PARIS, is since 27 years mainly devoted to production and irradiation technologies. To satisfy these objectives, OSIRIS is equipped by different test sections allowing mainly; 1) the long time irradiation of different materials including fuel rods, reactor vessel materials, fusion reactor components..., 2) the power ramps of fuel rods, 3) the Silicon doping, 4) the radioelements production, 5) the neutronography of materials and test sections. In most of the loops, the nuclear reactor conditions are fully simulated to approach as far as possible the exact behaviour of the materials. Through the new irradiation facilities under development, let's cite the OPERA test section foreseen for the simultaneous irradiation of 32 fuel rods with a maximum length of 2 m. To guarantee the safety and the high performances of the reactor, a continuous maintenance and improvement programme took place during the whole life of the reactor. The paper gives an overview of the part of this programme devoted to the replacement of the instrumentation and control system of the reactor. After 5 years study and development, the on site work took place in the second part of 1992 allowing a reactor start up beginning of 1993. (J.P.N.)