Abstract During its first year of service the m.h.d. experimental rig at Renardieres has been used for various studies on the performance of different sized m.h.d. generators and of the technology of electrodes and insulating walls. However, these experiments are not yet finished, and this report aims only at describing the present state of the most interesting studies and at comparing the results with those published by other research teams. The paper briefly describes the characteristics of the experimental plant and the conditions under which m.h.d. experiments have been carried out. Changes in the electrical and physical conditions of ionized gases during their passage through the test section have been studied, as a knowledge of these conditions is necessary for the correct interpretation of conditions in full size m.h.d. generators. Studies on m.h.d. generator materials have included insulating walls, ‘hot’ electrodes and ‘ cold ’ electrodes. However, only the studies on ‘ cold ’ electrodes are reported in detail as they are the most advanced to date; the studies on ‘ hot ’ electrodes have not progressed far, and for insulating walls, the use of cold ' peg walls ’ developed by Avco has been adopted. Finally some results of studies on generator operation are discussed.