The calcium-sensing receptor (CaR) is known to regulate how our cells respond to fluctuations in extracellular calcium levels. A report from John Wysolmerski and colleagues from Yale University in the February 16 issue of the Journal of Clinical Investigation reveals that in mice, at the transition from pregnancy to lactation, expression of CaR on mammary epithelial cells is upregulated. Dietary restriction of calcium upregulated parathyroid hormone-related protein production during lactation, decreased the calcium content of milk, increased milk osmolality and protein concentration, and decreased overall milk production. Most of these effects were prevented by the infusion of calcimimetic compounds, implicating CaR as the key mediator of these processes. This indicates that the lactating mammary gland can sense calcium increases and losses and adjusts its secretion of calcium accordingly. This helps match milk production with the availability of calcium in the mother's body.