While it is generally agreed that the pattern of growth of the condylar cartilage of the mandible can be affected by alterations in its immediate biomechanical environment, the precise nature of the factors involved is unclear. The proposed research will investigated the influence of lateral pterygoid muscle activity on chondrogenesis in the rat condylar cartilage by direct stimulation or inhibition of contractile activity in: 1) the lateral pterygoid muscle and 2) a functional synergist -- the superficial masseter muscle -- which has no direct attachment to temporomandibular joint structures. Chronic stimulation of muscles activity in patterns characteristic of tonic and phasic activity will be accomplished using indwelling fine wire electrodes connected to a stimulator. Inhibition of muscle activity will be produced by direct injection of myotoxic agents (bupivacaine, lidocaine with epinephrine). Mitotic activity and rate of mineralization in the condylar cartilage will be evaluated using histological, radioautographic, and biochemical (3H and 45Ca) techniques at sequential time intervals following each procedure. Adaptations within the lateral pterygoid muscle will be evaluated histologically and histochemically. These data should help resolve whether some aspect of the contractile activity of the lateral pterygoid muscle or the altered biomechanical circumstances resulting from its action (protrusion) is instrumental in the regulation of chondrogenesis. The results of this study should have direct relevance to the design of orthodontic appliances and should enable more accurate prediction of the effects of specific treatment modalities to be made.