 Two to three days after the sixth infusion, patients not achieving a 50 improvement of their MADRS score, when compared to their baseline score prior to the first of the six infusions, will be taken out of the study and offered to be treated in our Research Unit using additional pharmacological andor psychotherapeutic approaches. Patients improving by 50 or more on the MADRS scale will receive ketamine once a week for four weeks. They will be assessed using the same scales as mentioned above prior to each infusion. Patients will be assessed two to three days after the fourth weekly infusion. The ratings throughout phase III will help determine whether repeated administration of ketamine is effective to maintain the beneficial action of a relatively more intense regimen of ketamine. 