To determine the onset of subjective fatigue and the subjective endurance of fatigue and pains in the masseter muscle, ten adult male subjects exercised maximal voluntary tooth clenching. Concomitantly, the electrical activity in the masseter muscle was recorded by bipolar surface electrodes and integrated. Onset of subjective fatigue occurred on average 21 s after onset of muscle hyperactivity and the isometric endurance time of the masseter muscle, during which period severe muscle pains were present, was on average 82 s. The electrical muscle activity of the threshold and endurance tests showed a curvilinear configuration (y =a + b log x), and it is suggested that progressive physiological, or peripheral, muscle fatigue accompanied the psychological, or central, endurance of tooth clenching.