The effects of 20-min exposure to low-intensity, pulsed ultrasound were investigated in ST2 cells of bone marrow stromal origin. They responded to ultrasound with elevated levels of IGF mRNAs, osteocalcin, and bone sialoprotein mRNAs. The upregulated expression of these messages appeared in a biphasic manner, with the first peak resistant to the protein synthesis inhibitor cycloheximide, and a second peak that was eliminated by NS398, an inhibitor of the inducive prostaglandin G/H synthase (cyclooxygenase-2). A cumulative effect of mechanical loading called the memory effect, which has been observed in vivo, can be explained from such a biphasic anabolic reaction mediated by prostaglandins. The upregulation of IGF or osteocalcin mRNAs can be observed even at 24 h after the initiation of the 20-min exposure to ultrasound. Our results suggest that this low-intensity, pulsed ultrasound, which has been clinically used to accelerate the healing processes of fractured bone, induces a direct anabolic reaction of osteogenic cells, leading to bone matrix formation.