Mechanical forces regulate the function of bone cells. In this paper, the effects of cyclic stretching on osteoblasts derived from rat calvaria were studied at a magnitude occurring in physiological loaded bone tissue. A four-point bending apparatus was used to apply cyclic stretching on osteoblasts. Stretching at 500 me for 2-24h resulted in an increase in matrix synthesis(Po0:01). In contrast, the cyclic stretching at 1000 and 1500me for 2-24h inhibited osteoblast collagen production (Po0:01). We also described our newloading method to increase strain magnitude step-by-step. The strain magnitude increased by 500me increments from 500 to 1500me every 2 or 12h, respectively. Results showed that osteoblasts could absorb large amount of proline for collagen synthesis when stretched at 500me. However, not all the absorbed proline was used to synthesize collagen. Some of it was stored in cells. When the suitable signal (500me) was changed to an inhibiting signal (1000me), cells responded to it accordingly and released proline to medium. These results demonstrate that the response of osteoblasts is dependent on the magnitude of the strain applied and cells can adjust their bio-chemical response to adapt to the changing environmental stimulation.