The effect of implanting MgO paste into the bone marrow of rat tibia, was studied by light microscopy, time of flight-secondary ion mass spectrometry (ToF-SIMS), and environmental scanning electron microscopy (ESEM), and energy dispersive X-ray (EDX) analysis. After three weeks of implantation, the thickness of compact bone increased by 25% compared to sham-operated controls, while no effect was seen on the trabecular bone. In order to further elucidate the mechanism of the Mg-induced increase in bone mass, EDX and ToF-SIMS analysis of the bone samples was made at two weeks. At this time-point, no detectable difference in the thickness of the compact bone in Mg-treated and non-treated animals was observed. The Mg-content of the bone marrow and bone tissue of the Mg-exposed animals did not differ from that of sham-operated controls, implying that there are no traces of the implanted MgO when the mass of compact bone increases, between two and three weeks after surgery. The ratio of Mg/Ca content was higher in the bone of Mg-treated animals, indicating an altered structure of the bone mineral, which was confirmed by the ToF-SIMS analysis, showing increased levels of MgCO3, phosphate ions and CaF in the bone of MgO-exposed animals. Possible cellular activities behind the effect of MgO on compact bone thickness are discussed.