 1. To assess whether ketamine responders will improve further after a six infusions over two weeks. It is hypothesized that ketamine non-responders following a single infusion will not show a significant improvement with repeated infusions of ketamine at a dose of 0.5 mgkg given three times a week for two weeks, but that in responders repeated ketamine will produce a more robust and sustained antidepressant action than a single infusion. 