We investigated functional roles of the superior and inferior heads of the human lateral pterygoid muscle (SUP. LPT and INF. LPT, respectively). In 6 subjects EMG activities were recorded from the both heads of the lateral pterygoid muscle and other masticatory muscles. The movements of the incisor and condyle points were measured. In stepwise jaw opening and closing movements, the activity of the SUP. LPT was in inverse proportion to the extent of jaw opening. When the jaw performed hinge-like movement without anterior displacement of the condyle, the relationship between the SUP. LPT activity and the extent of jaw opening became linear, in both opening and closing phases. This indicates that the SUP. LPT activity is well correlated with the rotational angle of the mandible, whereas the activity of INF. LPT is proportional to the extent of anterio-posterior displacement of the condyle. The working range of the SUP. LPT was between 0 and 3 mm in anterio-posterior condyle displacement, in all different types of jaw movemnt in the same subject. The INF. LPT started to be active at 3 mm anterior to the posterior-most position. There was no overlap in their working range between the SUP. LPT and INF. LPT, indicating that they act as antagonist.