It has been known for more than a century that bone tissue adapts to functional stress by changes in structure and mass. However, the mechanism by which stress is translated into cellular activities of bone formation and resorption is unknown. We studied the response of isolated osteocytes derived from embryonic chicken calvariae to intermittent hydrostatic compression as well as pulsating fluid flow, and compared their response to osteoblasts and periosteal fibroblasts. Osteocytes, but not osteoblasts or periosteal fibroblasts, reacted to 1 h pulsating fluid flow with a sustained release of prostaglandin E2. Intermittent hydrostatic compression stimulated prostaglandin production to a lesser extent: after 6 and 24 h in osteocytes and after 6 h in osteoblasts. These data provide evidence that osteocytes are the most mechanosensitive cells in bone involved in the transduction of mechanical stress into a biological response. The results support the hypothesis that stress on bone causes fluid flow in the lacunar-canalicular system, which stimulates the osteocytes to produce factors that regulate bone metabolism.