This study was aimed to assess the effects of fluid shear stress on the bone resorption in rat osteoclasts. The osteoclasts were derived from the lumbar vertebrae marrow cells which were isolated from the 6-month-old female Sprague Dawley rats, cultured on the slide at 1 x 10(6) cell/ml, and induced with 1, 25 (OH)2 Dihydroxyvitamin D3. The slide containing osteoclasts was taken out on day 7 after culture and then put into the flow chamber. The loads of fluid shear stress applied to the osteoclasts were 5.97, 11.36, 16.08 and 20.54 dyne/cm2, respectively, for 30 minutes. The osteoclasts unloading fluid shear stress were used as control. The bone resorptive pits were studied by light microscopy and scanning electron microscopy. The tartrate-resistant acid phosphatase (TRAP) secreted by osteoclasts was detected with ultraviolet spectrophotometry. The results showed that fluid shear stress can increase the activity of TRAP and significantly increase the number and area of bone resorptive pits made by osteoclasts,and the effect of fluid shear stress on the bone resorption of osteoclasts is the same as that on the activity of TRAP. The reaction of the osteoclasts to the fluid shear stress in this study also suggested that the bone resorption of osteoclasts be increased in a magnitude of fluid shear stress-dependent manner, and that the changes of TRAP activity be closely related to the changes of the number and area of resorptive pits of the osteoclasts.