Mechanical strain produced by forward mandibular positioning was found to enhance mandibular condylar growth in experimental animals and in patients. This study was designed to identify the changes in number and rate of the proliferating mesenchymal cells in mandibular condyles of adult rats and to correlate these changes to the expression of SOX9 and type II collagen under mechanical strain. Seventy-eight 120-day-old female Sprague-Dawley rats were randomly allotted to six groups, nine animals in each experimental group according to different time points. Cell kinetic studies for expression of PCNA were used to identify number and rate of proliferating mesenchymal cells. Immunostaining of SOX9 and in situ hybridization of Col2a1 gene were carried out. Results showed a significant increase in number of replicating mesenchymal cells and proliferation rate. The expression of SOX9 was enhanced and Col2a1 gene transcript was then activated. The proliferative layer became thicker on experimental day 21. The thickness of chondroblast layer and chondrocyte layer showed significant increase from experimental day 14 to day 30. In conclusion, mechanical strain produced by mandibular advancement in adult rats promotes the proliferation of mesenchymal cells. Under control of transcription factor SOX9, these mesenchymal cells are then committed to enter the chondrogenic route leading to condylar growth.