The behavior of the fluid in stressed bone is examined in order to determine what stimuli cells in bone would experience when bone is stressed. It is found that the fluid pressure in the vascular channels should have a very short relaxation time, on the order of millisecondsin vivo. The case for the lacunar-canalicular systems is more difficult to analyze, but the fluid pressure in these pore systems may not relax as rapidly. The experimental evidence concerning fluid flow is difficult to interpret because of the complicated porosity in bone and the dependence of the permeability on sample preparation techniques. Nevertheless, the data are consistent with a fast relaxation time for the vascular channels in vitro, which means that the long relaxation time observed in the electromechanical effect is due either to the flow in the lacunar-canalicular systems or to the flow in the microporosity.