PTH induces the synthesis of insulin-like growth factor I (IGF-I) and regulates the expression of IGF-binding proteins (IGFBP) in osteoblast cultures. IGFBP-related protein-1 (IGFBP-RP-1), the product of the mac25 gene, binds IGF-I, IGF-II, and insulin. We tested the actions of PTH on the expression of mac25/IGFBP-RP-1 in cultures of osteoblast-enriched cells from 22-day-old fetal rat calvariae (Ob cells). PTH at 0.1-10 nm for 6-48 h increased mac25/IGFBP-RP-1 messenger RNA (mRNA) levels in Ob cells, an effect not altered by cycloheximide. PGE2 increased mac25/IGFBP-RP-1 mRNA levels, but indomethacin did not modify basal or PTH-stimulated mac25/IGFBP-RP-1 expression. The decay of mac25/IGFBP-RP-1 mRNA in trans-criptionally arrested Ob cells was not modified by PTH, and PTH increased the rate of IGFBP-RP-1 transcription. GH, insulin, bone morphogenetic protein-2, fibroblast growth factor-2, platelet-derived growth factor BB, IGF-I, and IGF-II did not modify mac25/IGFBP-RP-1 expression, whereas transforming growth factor-β1 was modestly stimulatory. In conclusion, PTH stimulates mac25/IGFBP-RP-1 transcription in osteoblasts, an effect that could be relevant to the actions of PTH in bone.