According to literature reports based on in vitro tests, transient increase in Mg2+ concentration to 2 to 10 mM can promote osteogenesis around implants. However, it is not well established how the Mg2+ concentration may change in space and time around a Mg2+-releasing implant in vivo, and whether therapeutic levels of Mg2+ can be maintained in the surrounding tissues. Thus, the temporal and spatial evolution of Mg2+ concentration was investigated from a MgSO4·7H2O containing polycaprolactone implant by numerical analysis using a rudimentary diffusion model. Diffusion coefficient of Mg2+ in polycaprolactone was determined experimentally, while it was estimated in body fluids and tissue from literature data. The results revealed that Mg2+ levels generally increase modestly and reach the therapeutic level for less than one day in this system due to fast clearing of Mg2+ from the tissues. Toxic levels of Mg2+ concentration may develop, but only for a few minutes, and can be mitigated by a thin layer of pure polymer applied on the implant.