Importance: Almost one third of patients fail to respond to standard antidepressants. Proof of efficacy of phenytoin augmentation in such cases would: 1) be a useful addition to our clinical armamentarium 2) strengthen the theoretical profile of phenytoin as an antimanic, antidepressant and mood stabilizing compound parallel to lithium and several other anticonvulsants 3) raise useful and heuristic ideas for mechanistic studies of phenytoin's effects on mood.