['Mechanical stress such as orthodontic forces can produce mechanical damage and inflammatory reaction in the periodontium. Osteopontin (OPN) is a multifunctional cytokine that has been correlated with periodontal disease progression. Because the periodontal ligament (PDL) can be affected by stress and PDL cells are involved in periodontal destruction and remodeling, we aimed to study the influence of mechanical stress on the expression and regulation of OPN in human PDL (HPDL) cells.', 'The mechanical stress was generated by continuous compressive force, and the expression of OPN was examined by reverse transcription-polymerase chain reaction and Western analysis. The application of inhibitors was used to examine the mechanism involved.', 'Both mRNA and protein expression of OPN significantly increased in a force-dependent manner. Increase of receptor activator of nuclear factor-kappa B ligand (RANKL) was also observed. Interestingly, application of indomethacin could abolish the induction of RANKL but not that of OPN, suggesting the cyclooxygenase-independent mechanism for stress-induced OPN expression. In addition, the upregulation of OPN was diminished by Rho kinase inhibitor but not by cytochalasin B.'] Does mechanical stress induce osteopontin expression in human periodontal ligament cells through rho kinase?