We analyzed the expression, during the osteogenic response of bone to mechanical stimulation, of insulin-like growth factor I (IGF-I), a growth factor implicated in bone formation, and c-fos, a protooncogene in which disordered regulation specifically affects bone. Both genes were strongly expressed in osteocytes of mechanically stimulated but not control bones within 30 min of the osteogenic stimulus. IGF-I mRNA expression increased up to 6 h, was restricted to osteocytes, and was strongly suppressed by indomethacin. Although early IGF-I mRNA expression was resistant to cycloheximide, there was a degree of suppression after 6 h, raising the possibility that IGF-I expression might be prolonged by autocrine mechanisms. c-fos mRNA was increased both in osteocytes and on bone surfaces. At both sites, c-fos expression was transient, prolonged by cycloheximide, and was strongly stimulated even in the presence of indomethacin. Thus osteocytes respond to mechanical stimulation with immediate prolonged expression of IGF-I and immediate transient expression of c-fos, implicating osteocytes in the osteogenic response to mechanical stimulation. Moreover, the different spatial distribution and indomethacin sensitivity of c-fos and IGF-I gene expression suggest that at least two signaling pathways are activated in osteocytes during this process.