Objective: To study the condylar mesenchymal cells and cartilage layers during continuous advancement of the mandible. Methods: seventy-eight 120-day-old female Sprageu-Dawley rats were allotted to six groups which were sacrificed on days 3, 7, 14, 21, 30 and 60, respectively. Each group comprised 9 rats with bit-jumping device and 4 controls. Immunostaining for PNCA was used to access the number of proliferating mesenchymal cells and proliferating rate. Alican Blue-PAS staining and immunostaining of type II collagen were used to distinguish and quantify the thickness of the cartilage layer. Results: The number of replicating mesenchymal cells increased on experimental days 3 and 21. The proliferation rate also increased on the same days. The proliferative layer became thicker on experimental day 21. The thickness of the fibrous layer also increased from day 21. The thickness of the chondroblast and chondrocyte layers showed a significant increase from experimental day 14 to day 30. Conclusion: Continuous mandibular advancement promotes profileration and induces recruitment of mesenchymal cells in the condyles of adult rats. Such cellular response ultimately results in new bone formation in the condyles of adult rats. This study was supported by The University of Hong Kong (CRCG grant 10203764.15633.08003.323.01).