Bone is known to alter its architecture, mass, composition, metabolic state, development and function in response to the effects of external forces applied to it, whether those forces be loading or unloading in nature (1). These adaptive changes by bone and the vertebrate skeletal system in general may be imparted by mechanical forces, gravity, buoyancy, or other such influences and manifest themselves in a number of ways. The skeleton of an animal, for example, may increase in mass through exercise and heightened activity, it may lose mass as a result of extended immobilization or weightlessness, or it may remodel during fracture repair processes. A mechanism explaining the adaptation by bone and the skeleton to the presence or absence of applied forces is not completely understood, but the changes are thought to occur ultimately at the cellular level of structure. Studies presented here have examined the relationship between forces and bone cell response in this context: Spaceflight and weightlessness have been utilized to investigate the influence of gravitational unloading on a model of cultured osteoblasts derived from normal embryonic chicken calvaria.