Magnesium (Mg) alloys are considered promising materials for biodegradable medical devices; however, the initial effects and distribution of released Mg2+ ions following implantation are unclear. This is addressed in the present study, using two types of Mg alloys implanted into rats. An in vitro immersion test was first carried out to quantify Mg2+ ions released from the alloys at early stages. Based on these data, we performed an in vivo experiment in which large amounts of alloys were subcutaneously implanted into the backs of rats for 1, 5, 10, and 25 h. Mg2+ accumulation in organs was measured by inductively coupled plasma mass spectrometry. In vivo, blood and urine Mg2+ concentrations were higher in rats receiving the implants than in controls after 1 h; however, the levels were within clinically accepted guidelines. The Mg2+ concentration in bone was significantly higher in the 25 h implanted group than in the other groups. Our results suggest that homeostasis is maintained by urinary excretion and bone accumulation of released Mg2+ ions in response to sudden changes in Mg2+ ion concentration in the body fluid in a large number of Mg alloy implants at the early stages.