Plasma total calcium levels have been studied in pregnant rabbits and their fetuses from 16 days gestation to term. Between 16 and 21 days gestation the observed fetal plasma calcium level falls from the maternal level or above to 1-6 mg/100 ml. below the maternal level. Thereafter, the fetal plasma calcium rises and at 30 days gestation exceeds the maternal level by 1*9 mg/100 ml. There is a high correlation between fetal and maternal levels at 21 days which is lost by 30 days. Fetal skeletal calcification was studied using an alizarin stain technique. The earliest centres of calcification were present in the clavicle and mandible at 16 days. It is concluded that the observed fall in fetal plasma calcium near mid-gestation may be due to the onset of skeletal calcification and the increase after 21 days to one or more of a variety of factors. Five 21 days pregnant animals were infused with 1 per cent calcium chloride solution (1 g anhydrous calcium chloride in 100 g aqueous solution) for up to 2j hr. The maternal plasma calcium concentration was elevated by 7.7 mg/100 ml. during the infusion but the fetal level was only raised by 2-0 mg/100 ml. The correlation between fetal and maternal levels was lost during the infusion. These findings are compatible with either slow calcium transfer across the placenta or fetal calcium homeostasis. It is well established in many mammalian species that during late gestation there is a greater concentration of calcium in fetal than in maternal plasma. In the guinea pig, on the contrary the fetal plasma calcium concentration at mid-gestation falls from about the maternal level (9.6 mg/100 ml.) to 5-5 mg/100 ml. and then slowly rises, reaching the maternal level again only near the end of gestation [Graham and Scothorne, 1970]. This paper presents a study of maternal and fetal levels, from a relatively early stage in gestation, in another species. The rabbit was chosen since there is already some evidence that fetal levels are lower than maternal at 21-23 days of gestation [McIsaac, 1927; Economou-Mavrou and McCance, 1958]. The stage in gestation when the homeostatic mechanism for calcium becomes mature in the fetus has not yet been identified, although there is experimental evidence that the parathyroid can elaborate hormone early in embryonic development [Scothorne, 1964: Graham and Scothorne, 1971]. In late gestation, when fetal calcium levels are higher than maternal, the fetal level remains constant when the level in the mother is elevated by infusion of calcium chloride [Bawden and Wolkoff, 1967: Greeson et al., 1968]. In this work, when low fetal levels earlier in gestation were substantiated, it was decided to study these levels after induction of maternal hypercalcaemia. METHODS The accuracy of the calcium estimation was assessed by preparing nine separate 5 ml. plasma solutions [Fawcett and Wynn, 1961] each containing 0 05 ml. of the same human plasma. The calcium content of each small sample of plasma was determined 160