Fatigue was developed by four human subjects after negative work, and by two subjects after negative/positive work by the jaw muscles. Another four subjects did not develop fatigue subsequent to muscular exercise. In subjects with fatigue, the ability to perceive and/or produce repetitively a predetermined mandibular posture, i.e. mandibular kinesthesia, was impaired by about 285%. It is suggested that the distorted kinesthesia resulted from either a fatigue-induced failure of, or a fatigue-induced persistence of, jaw muscle fibers to contract, which in turn led to distorting flows of afferent impulses from muscles spindles and/or tendon organs.