Physiological stresses such as heat stress, chemical stress and mechanical stress induce the expression of heat shock protein (HSP) families in cells, which affects cell function. In the present review, we describe HSP27, a small HSP in osteoblasts, especially the regulatory mechanism of the induction of HSP27 stimulated by physiological bone agents. Chemical stress by sodium arsenite (arsenite) induces HSP27 coupled to the metabolic activity of the arachidonic acid cascade, and the HSP27 induction by arsenite is negatively regulated by activation of protein kinase C (PKC). On the contrary, physiological regulators of bone such as endothelin-1, prostaglandin F2 alpha (PGF2 alpha), PGD2, and basic fibroblast growth factor (bFGF) induce HSP 27 via protein kinase C (PKC) activation. In addition, the mitogen-activated protein (MAP) kinase super-family takes part in the HSP27 induction. Thus, not only stress but also physiological agonists induce HSP 27 in osteoblasts, and PKC or MAP kinases play important roles in the induction of HSP27.