The project has two phases: a 12 week treatment phase and a 6 month follow-up phase. In the first phase, a double-blind one-way cross-over design will allow us to compare the clinical and physiologic responses to TNS at two frequencies of stimulation, "high" (~120 Hz) and "low " (~20 Hz). Subjects will be randomized to start a six week period at either high or low frequency stimulation (n=10 in each); both subjects and the staff who interact with them will be blinded to assignment, and a separate member of the team will program the device settings. At the six week point, the low frequency group subjects will be crossed over to high frequency, while the high frequency group will continue at high frequency. At the end of the 12 weeks, the TNS systems will be returned and the adjunctive treatment will end. All subjects will be followed for another six months with monthly telephone calls to monitor symptoms or changes in treatment that would signal a return of symptoms. The primary endpoint is the change in depression severity for the two groups at the week 6 visit.