Previous studies have investigated the relationship between prefrontal cortex activation and perceived exertion during prolonged exercise. However, the effect of perceived exertion on prefrontal cortex activity is confounded by exercise intensity. Therefore, the changes in prefrontal cortex activity in response to perceived exertion remain unclear. The purpose of the present study was to investigate the relationship between the activation (oxygenation) of the prefrontal cortex and perceived exertion during constant work-rate elbow-flexion exercise with or without muscle-spindle stimulation. Ten healthy, right-handed subjects participated in the study. Near-infrared spectroscopy with probes positioned over the prefrontal cortex measured its activation throughout elbow-flexion exercise. Subjects performed sustained elbow-flexion exercise at 25-35% of the maximal voluntary contraction (MVC) with or without muscle-spindle stimulation (vibration), which can decrease perceived exertion. The ratings of perceived exertion were significantly lower during exercise with vibration (Ex-Vib) than during exercise without vibration (Ex) (p<0.05). The oxygenation of the prefrontal cortex during Ex-Vib did not significantly differ from that during Ex (p>0.05). These results indicated that perceived exertion was not necessarily associated with prefrontal cortex activation during exercise.