Bone is a tissue that responds to mechanical load by changing its internal architecture. However, the mode of transmission of mechanical stimuli into biological signals and the effect of load at the cellular level are still not clear. An in vitro system, a Flexercell Strain Unit, was used to apply cyclic load to osteoblast-like cells in culture. In the first series of experiments, ROS 17/2.8 rat osteosarcoma cells, cultured on Flex I, flexible bottomed culture plates, were subjected to a 0.05 Hz, 0.24 STRAIN cyclic load regime for 3 and 7 days, in vitro. One group subjected to load received verapamil, a calcium channel blocker, throughout the experimental period. A second group was exposed to load but received no verapamil. A third group had no drug or load and a fourth group had no load but received verapamil. Cultures were incubated for 24 hours prior to collection with 10 microCi of 45CaCl in the medium, then well bottoms were divided to yield outer (maximum) and inner (minimum) load zones for assay of radioactivity. The effect of verapamil during a 7-day loading period was studied by adding the drug to individual cultures at daily intervals. Results indicated that mechanical loading stimulates calcium incorporation in ROS 17/2.8 cell cultures by day 7 but not by day 3. Only early verapamil addition decreased load-induced calcium incorporation when drug was added prior to day 4. If verapamil was added after 4 days, the channel blocker did not diminish load-induced calcium incorporation.