SUMMARY Plasma magnesium levels measured in rats from 16·5 to 21·5 days of gestation and during the first week after birth proved to be invariably higher in the foetus than in the mother. The highest level observed was in the 16·5-day-old foetus. A small decrease occurred between 16·5 and 17·5 days of gestation; thereafter the plasma magnesium level did not change until 19·5 days and then decreased between 19·5 and 21·5 days. After birth an increase in plasma magnesium occurred with suckling but then remained constant during the first week of life. Parathyroid hormone (0·25 USP unit/g) injected into 21·5-day-old foetuses had no effect on plasma magnesium levels from 0·5 to 24 h after injection. This dose was found to be very potent in raising plasma calcium values 4 h after injection. In the 3-day-old newborn rat this dose was similarly ineffective. Removal of the foetal parathyroid glands by decapitation at 17·5 days of gestation was followed by a decrease in plasma magnesium at 21·5 days of gestation. Parathyroid hormone (0·25 USP unit/g) injected into decapitated foetuses did not change the level of magnesium in the plasma. Salmon calcitonin (S-CT) at two doses (0·4 and 4 ng/g) produced no effect on plasma magnesium concentrations in 3-day-old newborn rats 3 h after injection; whereas at both doses, marked diminutions in plasma calcium and phosphate concentrations were observed. After injection of 40 ng S-CT/g, plasma magnesium decreased in 3-day-old newborn rats 3 h after injection. This dose was found to decrease plasma magnesium in the 19·5-day-old foetus and in the 20·5-day-old foetus. Before 19·5 days of gestation no effect was observed.