Avian calcium metabolism in an egg-laying period is extraordinary when compared with all other classes of vertebrates. The hard eggshell consists of calcium carbonate (CaCo3). About 60-75% of the calcium in the eggshell is derived from dietary sources and the remaining 25-40% from skeletal stores, called "medullary bone". Medullary bone is specifically developed in marrow cavities of long bones and plays an important role as a calcium reservoir for eggshell formation. An onset of medullary bone formation coincides with initiation of ovarian follicle maturation and the secretion of gonadal hormones, estrogen and androgen. Therefore their formation requires the combined influence of both estrogen and androgen. However, not only gonadal hormones but 1,25-dihydroxy vitamin D-3 is necessary for the formation of fully mineralized medullary bone. There are osteoblasts, bone-forming cells, and osteoclasts, bone-resorbing cells, on the hen medullary bone surface. In the domestic hen, medullary bone formation and resorption alternately occur during the 24-hour egg-laying cycle. These cyclic changes in medullary bone metabolism depend on the plasma levels of endogenous hormones fluctuating during the egg-laying cycle. Namely, when an egg is in the infundibulum, magnum or isthmus of the oviduct, osteoblasts actively form medullary bone with the induction of estrogen secreted by matured follicles. On the other hand, when an egg enters into the shell gland and begins to be calcified, the mobilization of calcium for eggshell formation causes a decrease of plasma ionized calcium concentrations. Consequently, to compensate the plasma calcium concentrations to be normal, parathyroid hormone is secreted from parathyroid glands to accelerate the osteoclastic bone resorption. With the end of eggshell formation, calcitonin is secreted from ultimobranchial glands thus inhibiting the osteoclastic bone resorption. In recent years, a number of manmade chemicals have shown to be able to mimic estrogen and androgen. It has been hypothesized that alterations in the normal pattern of endocrine systems, seen in some populations of wildlife, are linked with exposure to these chemicals. Interestingly, the medullary bone formation is induced by both estrogen and androgen, and the receptors of endocrine disruptors such as estrogen, androgen and aryl hydrocarbon (dioxin) receptors are highly expressed in the medullary bone. Therefore, it is sure that medullary bone would be a useful model to elucidate the effects of endocrine disruptors on wildlife and humans.