Fifty female SD rats of 4-week-old were selected, divided equally and randomly into experimental and control groups. Simulated functional appliances were used to guide the mandibles of the rats forwards. The animals were sacrificed after 3 days, 1 week, 2 weeks, 3 weeks and 4 weeks. The method of radioimmunoassay (RIA) was performed to quantitively detect the level of estrogen in the condylar cartilages. The results showed: 1. High levels of estrogen were detected in the condylar cartilages during their actively proliferative period. When they differentiated gradually, the levels of estrogen decreased. 2. The amount of estrogen was increased significantly after functional mandibular protrusion, especially during the period of the active proliferation of the condylar cartilages. It is suggested that estrogen has a close relationship with the hyperplasia and hypertrophy of the condylar chondrocytes and the increment of the levels of estrogen in the condylar cartilages will further promote the proliferation and functional adaptive remodelling of the condylar cartilages.