 2. Phase II: the repeated treatment over two weeks. Once patients are deemed not to have responded to the second infusion or have lost their improvement relapsing to a MADRS score over 80 of their baseline score prior to the second infusion, they will receive repeated ketamine infusions 0.5 mgkg. They will be given 3 infusions per week for two weeks. On visit 6 of phase 2 the patients will be assessed using the Brief Psychiatric Rating Scale-positive symptom scale106 at baseline before the infusion, time 0 immediately after infusion is complete, then at 60 and 120 minutes post time 0. Their clinical status will be assessed prior to each infusion using the MADRS, the CGI-S, the CGI-I, and they will rate themselves using the QIDS-SR and a self-report questionnaire rating suicidality. Patients will be assessed two to three days after the sixth infusion. The ratings throughout phase II will help determine whether repeated administration produces a greater and sustained improvement than a single infusion. Biological assays will be repeated at the end of Phase II. 