Bioactive glass powder in the MgO-CaO-P2O5-SiO2 system was mixed with water to create a bioactive glass paste. The paste was then placed in 8 cavities in molars of Sinclair mini-pigs, isolated using a light-cure composite filling, and left in vivo for 4 weeks. Additionally, 4 controls were run where the bioactive glass was placed in an inert polymer substrate and then incubated at 37°C for 4 weeks. Specimens were cut longitudinally in two halves and prepared for chemical and x-ray analyses. Qualitative results showed that the paste in the molars stayed intact while there was little or no paste left in the polymer substrate after cutting. This observation suggested that the paste in the natural tissue had structural integrity which could be caused by chemical changes and/or mineralization encouraged by contact with dentinal tubule fluid. X-ray analysis did not reveal any crystallinity in the paste at 4 weeks, but chemical alterations were confirmed by electron microprobe analysis. The chemical inhomogeneity of the individual elemental maps revealed the formation of Ca-P-rich/Si-poor areas. These distinct chemical variations were not seen in chemical analyses run on the bioactive glass paste in its initial state.