A reciprocal relationship between hostility and depression is postulated by psychodynamic theory. Following the classic formulations derived from Freud and Abraham, the turning of hostility away from the object world and onto the ego is a crucial mechanism in the psychogenesis of depression. The effects of imipramine and other antidepressants have been studied for possible verification of this hypothesis. Based on clinical observations and research studies, it is postulated that imipramine mobilizes hostility and that this mobilization underlies the drug's therapeutic psychodynamic action. In a study of psychodynamic changes in women with depression, opportunity was afforded to test this view. Three women, hospitalized for their depressions, were studied longitudinally before and during treatment with imipramine. Using the Gottschalk verbal sample technique, hostility directed inward and directed outward was measured. No significant differences were found between the imipramine and preimipramine periods, despite significant clinical improvement during the drug treatment period. Further longitudinal studies of the effects of imipramine upon hostility are needed, before the “mobilization hypothesis” can be accepted or dismissed.