Effects of interstitial fluid flow on osteoblasts were investigated. Intracellular cyclic adenosine monophosphate (cAMP) levels were monitored in cultured osteo‐blasts subjected to shear rates ranging from 10 to 3,500 sec⁻¹. Cyclic AMP levels were significantly increased at all shear rates from 1 pmole/mg protein to 10‐16 pmole/mg protein. Osteoblasts subjected to a shear rate of 430 sec⁻¹ for 0.5‐15 minutes exhibited elevated levels (12‐fold) of intracellular cAMP, which were sustained throughout the perfusion period. Osteoblasts were three times more sensitive to flow stimulation than human umbilical vein endothelial cells and baby hamster kidney fibroblasts, which also displayed higher cAMP levels (4‐fold) after exposure to flow. To distinguish streaming potential effects from shear stress effects, viscosity was increased 5‐fold by addition of neutral dextran to the perfusing medium. Shear stress is a function of viscosity, and streaming potentials are not for a given shear rate. The mechanism of this cellular response to flow was shown to be shear stress dependent. Inhibition of cyclooxygenase by 20 μM ibuprofen completely inhibited the flow‐dependent cAMP response, indicating the cAMP response is mediated by prostaglandins. Our results suggest that fluid flow induced by mechanical stress may be an important mediator of bone remodeling.