Twelve volunteers were investigated to determine their masseter muscle relaxation rate following voluntary contractions. Four of these volunteers were patients diagnosed as having a myogenous craniomandibular disorder. Electromyograms were recorded from the left and right masseter muscles and maximum bite force was recorded in the midline between the incisor teeth. A sustained contraction was maintained at 50% maximum voluntary bite force for 90 s, during which there was a brief relaxation every 10 s. Recordings were continued for a 3 min recovery period. This sequence was then repeated at 25% of maximum bite force. Median power frequencies were calculated from the power spectra for the first and last 3 s of the sustained contractions. Relaxation rates were measured for each brief relaxation during the sustained contraction and for the relaxation from each brief clench during the recovery period. It was found that maximum bite force values were very similar for volunteers in both the patient and control groups. Relaxation rates slowed more and percentage changes in median frequency were greater in the controls than in the patients during the sustained contractions. However, relaxation rates returned to initial levels more quickly in the controls than in the patients.