Reverse transcript polymerase chain reaction (RT-PCR) was developed for use in situ to measure mechanically mediated changes in gene expression activity in osteocytes within dense cortical bone. Using the functionally isolated turkey ulna model of bone adaptation, the left ulna of 6 old adult (36-40 months) male turkeys were subject to 4 weeks of a mechanical regimen consisting either of (1) 3000 microstrain at 1 Hz for 5 minutes/day or (2) 500 microstrain at 30 Hz for 10 minutes/day. The right ulna of each bird remained intact and served as control. Only a small percentage of osteocytes in the intact control bones and the 3000 microstrain ulnae showed any evidence of mRNA for collagen (each 1.2% +/- 0.3%). However, mRNA for collagen type I was strongly evident in 92.4% (+/-2%) of the osteocytes within the ulnae subject to the high frequency, low magnitude load. Sense primer control sections from both experimental and intact animals were used to verify that only osteocytes of the loaded bone had elevated the level of collagen mRNA. This high frequency, low magnitude mechanical stimulus was also sufficient to stimulate substantial new bone formation (14% +/- 5% over intact controls), whereas the low frequency, high magnitude stimulus failed to elicit any bone formation (-3% +/- 7%). These experiments show that specific mechanical regimens can activate the osteocyte's expression of a message responsible for the synthesis of proteins remote from the site where the formation of bone is ultimately to occur, even under systemic distress such as aging. Further, these data suggest that osteocytes perceive the strain environment and that they play a role in orchestrating the modeling/remodeling response. By developing a technique as flexible and powerful as RT-PCR for use in dense cortical bone, determining the relative contribution of specific proteins to the transduction of regulatory signals to formative or resorptive responses is facilitated.