Objective: To investigate the functions of the two heads of lateral pterygoid muscle in mouthopening and jaw-protruding resistance exercises with magnetic resonance imaging (MRI). Methods: Seven normal male subjects participated in the study. Four of the seven subjects did mouth-opening resistance exercises, while two did jaw-protruding resistance exercises and one did both mouth-opening and jaw-protruding resistance exercises over a period of two weeks. The oblique sagittal T2 -weighted MR images of lateral pterygoid muscle were obtained before and immediately after exercise and at 2, 4, 6, 8, 10, 15, 20 minutes after exercise. Signal intensity of the superior and inferior heads of lateral pterygoid muscle, cerebral grey matter, and masseter was also measured at these time points. Results: The signal intensity (SI) of the two heads of lateral pterygoid muscle in all the subjects was increased significantly and homogeneously after mouth-opening or jaw-protruding resistance exercise. The increased SI declined approximately to pre-exercise level in 20 minutes. No changes of SI were found in masseter and cerebral gray matter after exercises. Conclusion: The results of the present study strongly suggest that the two heads of the lateral pterygoid act synergistically and homogeneously during mouth-opening and jaw-protruding resistance exercise. The results do not support the concept that the two heads act antagonistically during jaw movements. Keywords: lateral pterygoid muscle, masticatory muscle, MRI, muscle recruitment, signal intensity