 Monitoring of the patients reaching more than 80 or more of their MADRS score, when compared to their baseline score prior to the last four infusions, will cease and will be offered to be treated in our Research Unit using additional pharmacological andor psychotherapeutic approaches. Patients maintaining more than 20 improvement will be assessed weekly to determine how long the therapeutic effects of ketamine last. Biological assays will be repeated at the end of Phase III. 