Recent preclinical and clinical research has demonstrated that the neuropeptide substance P (SP) plays a role in the central nervous system (CNS) response to stress, and perhaps in the etiology of major depression and/or anxiety disorders. The nature of this role, however, is poorly understood. A limited body of evidence suggests that in medication-free depressed patients, cerebrospinal fluid (CSF) concentrations of SP may be elevated relative to healthy controls. Two studies have shown that antidepressant treatment does not significantly change CSF concentrations of SP. Using standard lumbar puncture techniques, baseline CSF samples were obtained from 19 medication-free healthy controls and 19 medicated patients with treatment-resistant depression (TRD). Mean CSF SP concentration was significantly lower in TRD patients on psychotropic medications than in the group of healthy subjects. After 10-12 weeks of treatment with adjunct vagus nerve stimulation (VNS), CSF SP concentrations were not significantly changed. Low CSF SP may reflect a biological marker of the subtype of severe and chronic depression that is resistant to standard therapies.