PTH and other hormones that stimulate resorption affect osteoclasts indirectly by modulating cytokine production by osteoblasts. However, the identity and role of the osteoblast-derived cytokines involved in this process are unclear. To examine which cytokines are regulated by PTH, we assessed cytokine mRNA levels in osteoblasts using the reverse transcription-polymerase chain reaction technique. Of the 16 cytokines we examined, unstimulated MC3T3-E1 osteoblastic cells expressed mRNA for interleukins 5, 6, and 7, macrophage and granulocyte-macrophage colony-stimulating factors, transforming growth factor β1 and leukemia inhibitory factor. PTH specifically increased expression of interleukin-6 (approximately 50-fold) and leukemia inhibitory factor (approximately 10-fold). Levels of both IL-6 and LIF mRNA peaked 30–60 minutes after addition of PTH and returned to baseline by 4–6 h. This rapid and transient mRNA response, which resembles that of immediate early genes, was also observed in primary rat osteoblasts. The transient mRNA response was accompanied by increased secretion of IL-6 protein. Lipopolysaccharide, another stimulator of resorption, increased mRNA levels of a group of cytokines that were not induced by PTH, namely interleukin-1α, tumor necrosis factor α, and granulocyte-macrophage and granulocyte colony-stimulating factors. We conclude that osteoblasts produce complex networks of cytokines that (1) are regulated by bone-resorptive agents and (2) may be involved in controlling bone resorption.