The purpose of this research is to investigate the role of the adductor muscle comples of the mandible in determining the growth and form of the craniofacial skeleton. Emphasis will be placed on (a) defining the normal structure and function of the adductors of the mandible (i.e., the temporalis, masseter and medial pterygoid muscles) and the distribution of bone strain within the craniofacial skeleton as these covary during the growth process in rhesus monkeys (Macaca mulatta): and (b) determining the interrelationships between muscle function, bone strain and craniofacial form by experimental intervention in the adult. Analytical techniques will include gross and microscopic anatomy to determine muscle morphology, fiber length and cross-sectional are; electromyography to determine patterns and intensity of muscle activity; strain gauge analysis to evaluate stress distribution in the craniofacial skeleton in response to jaw function; contractile property analysis to determine, among other variables, length-tension relationships and fatigability; and histochemistry to determine fiber type. This research on the normal growth and function of the mandibular adductors in sub-adults, and adaptations following experimental intervention of facial form and function in adult monkeys, will provide a foundation for addressing such problems as orthodontic and surgical intervention for correction of gross maxillomanidbular disharmonies in growing children.