Mandibular condyles translate back and forth during mouth closing and opening in primates and most other mammals. To account for the functional significance of this phenomenon, several hypotheses have been proposed. The sarcomere-length hypothesis holds that condylar translation provides a mechanical advantage by minimizing sarcomere-length changes in the masseter-medial pterygoid complex throughout a wide range of jaw openings. As the hypothesis is inherently associated with the locations of the instantaneous centers of rotation (ICRs) of the mandible, a more accurate determination of this variable would help test this hypothesis. This study investigated ICRs in the sagittal plane during human symmetrical mandibular opening based on a recently developed analytical method. The results confirmed that, with inter-and intraindividual variation, the natural opening was a simultaneous rotational and translational motion. In addition, the ICR was found to lie closer to the condyle during the first 10Β°than during the rest of the rotation. This suggests that for the condyles the rotational component is somewhat more significant at the early phase than at the late phase of the opening stroke. For the whole range of the natural opening, the grossly approximated centers of rotation (CRs) scattered below the palpable lateral condylar poles in the superior half of the ramus. This study supports neither the ICR path determined by Grant ([1973], J. Biomech. 6:109-113) nor the conclusions reached by recording manually operated jaw movements in human cadavers (Rees [1954] Br. Dent. J. 6:125-133). Moss's suggestion ([1960] Disorders of the Temporomandibular Joint (Philadelphia: W.B. Saunders), pp. 73-88) that the center of rotation lies at the lingula is also not confirmed. Although the new data cannot reject the sarcomere-length hypothesis, they do not strongly support it either. Another hypothesis is proposed in this study as plausible. With this hypothesis, translation is regarded as an adaptation to the use of the inferior head of the lateral pterygoid as a jaw depressor in noncarnivorous mammals. Potential functional advantages of this portion of the muscle are also discussed.