Secretion of parathyroid hormone-related protein (PTHrP) by sheep fetal parathyroid glands is reported to be an important factor in the maintenance of a placental calcium pump. The aim of the present study was to determine whether the developing rat parathyroid glands express PTHrP or parathyroid hormone (PTH), or both. Hybridisation histochemistry was used to detect transcription of PTHrP and PTH in serial paraffin sections through the 12.5- and 13.5-day rat embryo parathyroid anlage, as well as in sections through the 17.5-day embryonic and adult parathyroid glands. Results show strong expression of PTH in the 13.5-day embryonic parathyroid anlage, as well as in the parathyroid gland of the 17.5-day embryo and adult. Transcription of the PTHrP gene was not detected. The more sensitive technique of reverse transcription PCR was then performed. The pharyngeal region of 11.5-, 12.5- and 13.5-day rat embryos was dissected out and, at each stage, RNA was extracted from these tissues, as well as pooled tissues from the rest of the embryo. RNA that had been extracted from adult thyroid/parathyroid tissue was also tested. After reverse transcription, the resulting cDNAs were amplified by PCR (50 cycles) using specific PTH and PTHrP primers. The results show an abundance of PTH mRNA, specific to the pharyngeal region of the 13.5-day embryo, as well as to adult thyroid/parathyroid tissue. PTHrP expression was detected at very low levels in both parathyroid and extraparathyroid tissues. The presence of immunoreactive PTHrP and immunoreactive PTH in the pharyngeal region and rest of the body of 12.5- and 13.5-day rat embryos was assessed by specific RIAs. Whilst immunoreactive PTHrP was not detected in any of the tissues assayed, immunoreactive PTH was detected only in the pharyngeal region of the 13.5-day embryo. This confirms the results obtained from the gene expression studies. We conclude then that, in the developing rat embryo, PTH rather than PTHrP is more likely to play a role in calcium regulation. This is in contrast with the reported situation in the sheep, and suggests that fundamental species differences in fetal calcium regulation exist in mammals.