Fourteen subjects were evaluated by needle electromyography in a trapezius myofascial trigger point and simultaneously in adjacent nontender trapezius muscle fibers during a control condition (forward counting), a stressful condition (mental arithmetic), and resting baselines. Based on recent data implicating autonomic innervation in muscle function, we hypothesized that the trigger point would be more responsive than the adjacent muscle to psychological stress. The results showed increased trigger point electromyographic activity during stress, whereas the adjacent muscle remained electrically silent. These results suggest a mechanism by which emotional factors influence muscle pain. This may have significant implications for the psychophysiology of pain associated with trigger points.