Our past results have shown that the placenta of an intact twin lamb fetus transports calcium more rapidly in vivo than that of its twin that had been totally nephrectomized 8 to 19 d earlier. Nephrectomy reduces the calcitriol concentration in the fetal circulation. The addition of calcitriol to the medium used to perfuse the placenta of a previously nephrectomized fetus rapidly increased the rate of calcium transport. We are now focusing more narrowly on potential individual components of this calcium transport process, which is fundamental to successful long bone growth and production of a viable neonate. Initially, we assessed the effects of calcitriol on the entry of calcium ions into the maternal-facing surface of discs of interplacentomal chorion in vitro, and our preliminary results clearly indicate the value of this new methodology in dissecting the basic mechanism of placental calcium transport. The chorion was obtained from fetuses between 120 and 145 d of gestation that had been nephrectomized 6 to 21 d earlier. These fetuses included 5 pairs of twins, 1 member of each pair of which had been nephrectomized. Our results demonstrate that previous nephrectomy, with subsequent reduction in calcitriol and development of fetal hypocalcaemia, was associated with a significant reduction in the rate of uptake of calcium ions into the maternal-facing aspect of the chorion. The addition of 600 pM calcitriol to the fluid bathing this chorionic surface led to a rapid, significant increase in the rate of uptake of calcium ions by the chorionic trophoblast epithelia cells. No such effect was noted with chorion from the placentas of fetuses with intact kidneys. The addition of diltiazem (50μM) to the incubation medium caused a reduction in the uptake of calcium ions by chorionic discs from either intact or nephrectomized fetuses. These results indicate for the first time that the ovine placental trophectodermal barrier is at least partly under calcitriol control and demonstrate the value of separation of a complex process into its constituent parts.