The ontogeny of parathyroid hormone (PTH) and PTH-related protein (PTHrP) gene expression was studied by hybridization histochemistry in the rat at various stages between implantation and full term. PTHrP mRNA was demonstrable in the early post-implantation trophoblastic giant cells but disappeared from this site before 13·5 days. Localized gene expression, detectable by the in-situ technique, began between 12·5 and 15·5 days in embryonic tissues. The distribution of gene expression suggests that PTHrP may be concerned with the process of implantation. Its widespread, yet clearly localized, distribution in embryonic and fetal tissues is consistent with a paracrine or autocrine function which may relate to the transforming growth factor-β family of growth factors. PTH expression occurred solely in the parathyroid and was detectable in the fetal parathyroid at 13·5 days of gestation.