The mechanism of calcium transport across the chick chorioallantois has been studied in vivo. The 15 day old membrane contains no calcium binding protein (CaBP) and attempts to stimulate transport by adding this protein or vitamin D metabolites did not enhance the normal transport process. The mitochondrial and microsomal fractions of transporting cells contain only a small percentage of the 45Ca actually moved across the epithelium and simultaneous measurements of 45Ca and 3H inulin indicate that the bulk movement of fluid is not involved in calcium transport. Calcium appears to be moved after being bound to a protein since strontium and PCMBS both interfere with transport and the membrane shows saturation kinetics. It is suggested that none of the existing theories provides an adequate explanation for the mechanism of calcium transport and some support is given to the possibility that intercellular routes are involved in part of the process.