Lactating mammals must supply large amounts of calcium to the mammary gland where it is transported across mammary epithelial cells and into milk. This demand for calcium is associated with transient loss of bone mass, triggered, in part, by the secretion of parathyroid hormone-related protein (PTHrP) from the mammary gland into the circulation. The calcium-sensing receptor (CaR) is a G-protein-coupled receptor that signals in response to extracellular calcium ions. It is responsible for coordinating calcium homeostasis by regulating parathyroid hormone secretion in the parathyroid glands and by regulating calcium handling in the renal tubules. Previous studies had shown that the CaR is expressed on mammary epithelial cells during lactation, and it had been suggested that CaR signaling in the mammary gland helps to coordinate its production of PTHrP and calcium transport into milk. In this study, we examined mammary gland PTHrP production and calcium transport in CaR(+/-) mice, a genetic model of CaR insufficiency. We found that haploinsufficiency for the CaR resulted in increased PTHrP production both in vivo and in vitro. In contrast, CaR haploinsufficiency impaired calcium transport into milk in vivo and transepithelial calcium transport by mammary epithelial cells in vitro. These data provide genetic confirmation that the CaR regulates PTHrP production and calcium transport in the lactating mammary gland. This allows the mammary gland to become a calcium-sensing organ and to participate in systemic calcium homeostasis during lactation.