Objective To investigate the change of the ultrastructure and the variation of synthesis in aggrecan,the main proteoglycan component of the cartilage matrix of mandibular condylar chondrocytes(MCC), under the mechanical pressure. Methods In vitro cultured MCC from two-week-old New Zealand rabbits were incubated under continuous pressure of 90 kPa for 60min,360min by hydraulic pressure controlled cellular strain unit.The ultrastructure of MCC was observed under transmission electron microscope(TEM).In situ hybridization method was used to test aggrecan mRNA expression in MCC. Results The appearance of MCC on TEM showed that the cells of the control developed in the normal way.After been pressed under 90kPa for 60min,the cells showed elongated and voluminous cellular processes with increasing aggrecan mRNA.When the pressure lasted to 360 min,some of the cells showed distinct sign of apotosis and the aggrecan mRNA decreased but still higher than that of the control. Conclusion Continuouspressure could cause changes on the ultrastructure and function of MCC.Pressure may be used to up-regulate the synthesis of aggrecan.