Interleukin (IL)-6, a cytokine produced by skeletal cells and known to increase bone resorption, has mitogenic effects for bone cells, possibly by regulating the synthesis of other local factors. We tested the effects of IL-6 and its soluble receptor (IL-6sR) on the expression of insulin-like growth factor (IGF)-I and IGF-II in cultured osteoblast-enriched cells from fetal rat calvariae (Ob cells). IL-6 did not modify IGF-I messenger RNA (mRNA) levels, but when tested in the presence of IL-6sR, IL-6 at 1 to 100 ng/ml increased IGF-I transcripts by up to 3.2-fold after 24 h. IL-6sR caused a small increase in IGF-I mRNA levels when tested alone. IL-6 and IL-6sR increased immunoreactive IGF-I levels by 2.4-fold after 24 h and 6.4-fold after 48 h. Cycloheximide prevented, and indomethacin markedly decreased, the effect of IL-6 and IL-6sR on IGF-I mRNA levels, but hydroxyurea did not. IL-6 and IL-6sR did not alter the decay of IGF-I mRNA in transcriptionally arrested Ob cells, and the half-life of the predominant 6.5-kb IGF-I transcript was about 11 h in control and treated cells. In addition, IL-6 and IL-6sR increased the levels of IGF-I heterogeneous nuclear RNA. IL-11 also increased IGF-I mRNA levels, whereas oncostatin M and leukemia-inhibitory factor did not. In contrast to their effects on IGF-I, IL-6 and IL-6sR caused only a modest increase in IGF-II mRNA and polypeptide levels. In conclusion, IL-6, in the presence of IL-6sR, increases IGF-I synthesis in Ob cells; this effect may lead to a secondary increase in bone formation.