This study of manic-depressive psychosis is limited to the close examination of the circumstances in which the transition from one state or phase of the illness to another occurred. By way of clinical illustration this study concludes that both the mania and the incapacitating depression are regressive defenses selected by the ego to cope with aggression aroused by the experience of object loss, and that the success or failure in a flight to a new object in the face of loss played an important role in the choice of the defense. To keep within the limited goal, this study concerns itself neither with the structure, or the genesis of the structure, of the manic-depressive defenses, nor with the nature of the management of transference and countertransference problems.