Chronic low back pain (CLBP) is associated with attentional deficits. The goal of the study was to determine if stimulation with transcutaneous electrical nerve stimulation (TENS) modulated pre-attentional (measured by P50 Auditory Evoked Potential [AEP] studies), attentional (measured by Psychomotor Vigilance Task [PVT]) and frontal lobe function (determined by relative frontal lobe blood flow measured using Near Infrared Spectroscopy [NIRS]) in subjects with CLBP. We hypothesized that TENS would have a beneficial effect on the dysregulation of pre-attentional and attentional processes in patients with CLBP. Four subjects with CLBP only, 3 with CLBP and depression (DEP) and 3 with CLBP and post-traumatic stress disorder (PTSD) were recruited. The TENS electrodes were place over the lumbar region. Stimulation lasted 20 minutes. P50 AEPs were recorded prior to the EA stimulation and every 10 minutes for 50 minutes. The PVT was measured prior to and 30 minutes after the stimulation. NIRS was measured continuously throughout the stimulation and for 30 minutes after the stimulation. The ANOVA was used to determine when statistical significance (pē0.05). Results: P50 AEP amplitude decreased (decreased arousal level) during TENS in patients with CLBP only and CLBP+PTSD; P50 AEP habituation decreased (less sensory gating) during TENS in patients with CLBP+PTSD. PVT minimal responses (optimal response) increased in CLBP+DEP but decreased (improved) in CLBP only and CLBP+PTSD and lapse domain (slowest 10% of trials) increased after TENS in CLBP+PTSD. Relative NIRS values decreased with TENS in patients with CLBP+DEP but increased in patients with CLBP+PTSD. In patients with CLBP with and without PTSD, TENS 1) has a calming effect as determined by decreased P50 amplitude and P50 habituation; and 2) allows increased reaction time performance, perhaps by decreasing frontal lobe blood flow by allowing attentional mechanisms to target PVT performance. TENS has different effects on CLBP patients with co-morbidities.