The purpose of this study was to determine whether the production of interstitial collagenase mRNA in response to parathyroid hormone (PTH) changes with osteoblast phenotypic development. To accomplish this, cells derived from fetal rat calvaria were examined. The calvarial osteoblasts, which proliferate when placed in culture, can be made to differentiate after confluence. Studies were performed on cells while they were proliferating, at confluence, and during the differentiation process. The cells were treated with PTH for various times, and interstitial collagenase mRNA was quantified by RNase protection assay. We concluded that the ability of PTH to induce interstitial collagenase mRNA in these cells increased with osteoblast phenotypic development. We also determined that the response could be mimicked by combining the effect of 8-bromo-cAMP and 12-O-tetradecanoyl-phorbol-13-acetate, stimulators of the protein kinase A and protein kinase C pathways, respectively, both known to be activated by PTH. The binding of nuclear factors to two regions previously reported to be important for PTH induction of the gene in UMR 106-01 cells was also examined. These data indicated that the binding of nuclear factors to oligonucleotides encompassing the TRE (-51) or the PEA3 (-80) elements changed with development of the osteoblast phenotype. The latter was also shown to be PTH responsive.