Osteocytes have been proposed to be the cells primarily responsible for sensing the effects of mechanical loading in bone. Osteocytes respond to loading in vivo, and have been shown to express osteotropic agents and their receptors, and cell/matrix adhesion molecules in vitro, but the functional significance of such findings is not clear. One obstacle to increased understanding of the role of osteocytes in the regulation of bone mass is that the cells are not easily accessible for study. In situ studies are difficult, and although it is possible to extract and culture osteocytes from neonatal bones, the responses of such cells might be very different from those in older bones in situ. We have developed a technique to investigate osteocyte gene expression in vivo, using the reverse transcriptase linked polymerase chain reaction (PCR), and have shown that they express mRNA for β-actin (β-ACT), osteocalcin (OC), connexin-43 (Cx43), insulin-like growth factor I (IGF-I), c-fos, and c-jun, but not tumor necrosis factor alpha (TNF-α) or tartrate-resistant acid phosphatase (TRAP). The principle behind the method is that after removal of the periosteum, tangential cryostat sections of a tubular bone contain RNA only from osteocytes and a very small number of endothelial cells as long as the marrow cavity is not broached. Using this method, we have investigated gene expression in cells from rat ulnar cortical bone under forming and resorbing bone surfaces. In addition, we have investigated the effect on gene expression of mechanical loading which, if repeated daily, initiates new bone formation on quiescent or resorbing surfaces. Although the expression of the genes we have studied in osteocytes is different from those expressed by the periosteal surfaces overlying the cortex, we have not detected loading-related changes in osteocyte gene expression in any cortical bones. This may be because of the extreme sensitivity of the PCR technique which can only resolve large differences in expression. The use of quantitative methods in the future may allow demonstration of regulated gene expression in osteocytes.