Calcitonin (CT) and ionic calcium (Ca++) were measured in paired serum samples from the umbilical artery and vein of 47 normal term babies (28 females and 19 males). In the whole group, we found higher CT levels in the vein than in the artery (P less than 0.01). Considered by sex, significant CT (P less than 0.01) and Ca++ (P less than 0.05) gradients existed only in the female group. CT and Ca++ were also measured in serum samples from a group of 26 normal babies (16 females and 10 males) at 2 days of life and in a group of 25 normal babies (12 females and 13 males) at 30 days of life. At 2 day we found a significant increase of CT (P less than 0.001) (females P less than 0.001 and males P less than 0.05), and a significant decrease of Ca++ (P less than 0.001) (females P less than 0.001 and males P less than 0.001), in umbilical artery blood levels at 30 days. Our data show more CT coming from the placenta to the foetus than vice versa, more evident in the females, suggesting an extrathyroidal source of CT, which may be the human placenta. The increase of CT at 2 days, opposing the release of calcium from bone, may contribute in part to the decrease in Ca++ levels observed at that time.