The relative fetal autonomy for plasma calcium in regard to the maternal plasma value led us to investigate a possible effect of maternal hormones on the placental transfer of calcium. For this purpose we have used pregnant Rats deprived of endogenous calcitonin (CT) by thyroidectomy (TX) or thyroparathyroidectomy (TPTX). Increases in fetal plasma calcium of 0.87 mg/dl for TX mothers, 2.46 mg/dl for TPTX mothers, and 0.54 mg/dl for normal mothers occurred at the end of a 1 hr. maternal calcium infusion 3.6 mg/100 g body weight) at 21.5 days of gestation. The absence of CT in TX and TPTX mothers and the already known reduced levels of placental Calcium-binding protein (CaBP) in TPTX Rats might explain these results. Maternal CT seems to have an inhibitory effect on the placental transfer of calcium, whereas placental CaBP might play a more important role as a "buffer".