AbstractMagnesium (Mg2+) is the second most abundant ion in the body but its regulation is poorly understood. Mg2+ deficiency is known to interfere with the physiological regulation of other electrolytes, such as calcium (Ca2+) and phosphate (P_i), and a number of hormones have been implicated in mediating such disturbances. Here we use a mouse model to understand how these changes occur over time. We show that in as little as one day, mice experienced a dramatic decrease in Mg2+ and Ca2+ excretion in urine. At the same time, Pi excretion was significantly increased. Given the speed of these changes, it is unlikely that FGF-23 regulates the increase in P_i excretion. Instead, other factors such as PTH may have a predominant role during the early stages of hypomagnesaemia. Our ultimate goal is to unravel the precise mechanism by which Mg2+ deficiency affects the regulation of Ca2+ and P_i, and understand the involvement of hormones such as PTH, FGF-23 and vitamin D in mediating these changes.