Objective To investigate the effect of gradually induced disordered occlusion（GIDO） on the expression of osteoprotegerin（OPG） and receptor activator of nuclear factor-кB ligand（RANKL） in mandibular condylar cartilage of rats. Methods Totally 48 rats, aged 8 weeks were included, and were divided into experimental and control groups randomly at 4 time points, with same gender distribution（n=3）. By inserting elastic rubber band the right side mandibular first molar and the left side maxillary first molar were moved mesially. Four weeks later, the right side mandibular third molar and the left side maxillary third molar were moved distally with same method. In this way, the GIDO was established in rats. The rats were sacrificed at the end of 2th, 4th, 6th and 8th week respectively after the application of the GIDO. The expression of OPG and RANKL in condylar cartilage was examined with immunohistochemical method and calculated by the area of positive cell percentage. Results OPG and RANKL expressed predominantly in condylar cartilage hypertrophic layer. The rats in experimental group expressed a higher OPG level in all of the 4 time points than their age-matched controls（P
