Objective: The aim of this study was to evaluate the progress of dentin caries under resin composite and glass-ionomer cement restorations in vivo. Method and materials: Sixteen rats were subjected to oral inoculation of the bacterial strain, Streptococcus mutans, and experimental dental caries was induced in the rats' molars. The dental caries induced in rat molars was sealed with resin composite or glass-ionomer cement without removal of the caries; the depth and width of bacterial penetration in the lesion were measured from histopathologically stained sections. Inflammatory cell infiltrations within the pulp were also examined. Results: Both bacterial penetration into dentin and caries spread were significantly reduced by sealing with glass-ionomer cement or resin composite. No significant differences in bacterial penetration and caries spread were seen between the sealing materials. Regarding the pulpal reactions, moderate to severe inflammatory cell infiltration was observed even in the sealed teeth. Conclusion: The caries lesions could not be completely arrested by sealing alone, although their progress was slowed from an active to a chronic status.