Subjects may be included in the study only if they meet all of the inclusion and none of the exclusion criteria. The investigator and Aventis must approve any waiver of these inclusion and exclusion criteria on a case-by-case basis prior to enrolling the subject. Both Aventis and the investigator must document this. No subject will be allowed to enroll in this study more than once. The essential inclusion criteria for entry are subjects who have at least moderate level of major depression (HAM-D = 20) who do not have psychotic depression and are resistant to treatment based on both retrospective and prospective rating are treatment refractory to specified antidepressants. Subjects are excluded who have failed antidepressants of similar mechanism to M100907. Female subjects must use two forms of birth control and subjects will be excluded based on medical criteria outlined in the full list of inclusion and exclusion criteria.