Background Receptor activator of nuclear factor-kappa B ligand (RANKL) and osteoprotegerin (OPG) are well-documented potent regulators of osteoclast development. However, their effects in mature bone cells and in organ cultures have not been well studied. It is uncertain whether their activities in different experimental models are comparable.Materials and methods RANKL and OPG were evaluated for their activities in mouse calvarial organ cultures, mouse bone marrow cultures, isolated rat mature osteoclast assays and rat primary osteoblast cultures.Results In murine calvarial organ culture, both muRANKL (>= 10 ng mL(-1)) and rRANKL (>= 100 ng mL(-1)) significantly stimulated Ca-45 release, while OPG (>= 50 ng mL(-1)) was an inhibitor of bone resorption. Meanwhile, [H-3]-thymidine incorporation in this assay was also modulated (indicating proliferation increases in the osteoblast lineage of cells) although these peptides had no direct effect on [H-3]-thymidine incorporation in isolated osteoblast assays. In mouse bone marrow cultures, muRANKL (>= 1 ng mL(-1)) and rRANKL (>= 5 ng mL(-1)) significantly stimulated osteoclastogenesis. The number of nuclei per osteoclast was also significantly increased. OPG strongly inhibited this index, with over 90% suppression at 1 ng mL(-1). Both muRANKL (10 ng mL(-1)) and rRANKL (100 ng mL(-1)) stimulated, while OPG (10 ng mL(-1)) inhibited osteoclast activity in isolated mature osteoclast assays.Conclusion The current study demonstrated that bone resorption modulated by RANKL and OPG, in murine calvarial organ culture, leads to changes in osteoblast proliferation, suggesting a feedback mechanism from osteoclasts to osteoblasts. In addition, it was found that RANKL and OPG have more potent effects on osteoclastogenesis than on the activity of mature osteoclasts.