Controversy exists over whether the increase in maternal serum parathyroid hormone levels observed during the second half of pregnancy is due to autonomous parathyroid function or is secondary to changes in maternal scrum ionized calcium levels. In order to study this problem further, 9 subjects were followed serially throughout normal pregnancy. Total scrum calcium, ionized calcium, parathyroid hormone (PTH), calcitonin, and albumin levels were measured monthly. Six of these subjects had the studies repeated 6 weeks postpartum. Scrum ionized calcium levels were found to decrease from 3.81 ± 0.12 mg/dl to 3.63 ± 0.18 mg/dl between 21 and 25 weeks' gestation. This decrease was significant at P < 0.01. The ionized calcium remained in this lower range until term. A significant return to 3.77 ± 0.1 mg/dl was observed 6 weeks postpartum. Serum PTH levels showed a significant rise after 21 weeks' gestation (P < 0.05). No serial change in serum calcitonin was observed during pregnancy, although the mean level of the group was significantly higher than in nonpregnant controls (P < 0.01). The increase in maternal serum PTH observed during pregnancy appears to be due in part to a decrease in maternal serum ionized calcium.