SUMMARY Measurable levels of immunoassayable parathyroid hormone (PTH) were present in the plasma of two out of five sheep foetuses of 80–142 days gestational age, and in three of the mothers at Caesarian section. There was no correlation between foetal and maternal concentrations although the range of values was similar. In response to foetal hypocalcaemia induced by EDTA infusion the plasma PTH concentration rose in all of the foetuses and there was a highly significant inverse correlation between the foetal plasma calcium and PTH levels. During the period of foetal hypocalcaemia the maternal plasma PTH levels did not change significantly. EDTA was also infused into three adult ewes. There was a similar rise in the concentration of PTH in the plasma of these ewes and it was concluded that the level of responsiveness of the foetal parathyroid gland to plasma calcium concentration was similar to that of the adult. The parathyroid glands would appear to be active from an early stage in foetal life and responsive to changes in serum calcium level.