Mechanical stress produces flow of fluid in the osteocytic lacunar-canalicular network, which is likely the physiological signal for the adaptive response of bone. We compared the induction of prostaglandin G/H synthase-2 (PGHS-2) by pulsating fluid flow (PFF) and serum in osteocytes, osteoblasts, and periosteal fibroblasts, isolated from 18-day-old fetal chicken calvariae, A serum-deprived mixed population of primarily osteocytes and osteoblasts responded to serum with a two- to threefold induction of PGHS-2 mRNA Serum stimulated PGHS-2-derived PGE, release from osteoblasts and osteocytes but not from periosteal fibroblasts as NS-398, a PGHS-2 blocker, inhibited PGE(2) release from osteocytes and osteoblasts with 65%, but not that from periosteal fibroblasts, On the other hand PFF (0.7 Pa, 5 Hz) stimulated (3 fold) PGHS-2 mRNA only in OCY, The related PGE, response could be completely inhibited by NS-398. We conclude that osteocytes have a higher intrinsic sensitivity for loading-derived fluid flow than osteoblasts or periosteal fibroblasts,