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Statement of problem. Exposing the tooth structure to chemicals used in displacing the gingival tissue is inevitable. Purpose. This study was undertaken to microscopically examine the effect of tannic acid on the prepared dentin surface. Material and methods. Forty recently extracted intact human teeth were mounted for buccal surface preparation. Teeth were equally divided into 10 groups. One group was kept as a control and the other teeth received a topical application of 15%, 20%, and 25% aqueous tannic acid solutions each for 5, 10, and 15 minutes at room temperature. All samples were washed, air-dried, then prepared for scanning electron microscopic examination. Two different areas in the cervical region were randomly selected and examined under magnifications of ×1000 and ×2000, then photographed. The assessment of the changes was performed qualitatively. Results. The results showed an incomplete removal of the smear layer in all experimental samples. The astringent action of the tannic acid solution on both the smear layer and the surface peritubular dentin around the orifices of dentinal tubules had contributed to their constriction. The degree of constriction of the orifices of dentinal tubules increased as higher concentrations of tannic acid solutions were used and as the application time was further increased at each concentration. Conclusion. The tannic acid had an incomplete action on the removal of the smear layer. It also seemed to have an astringent action on both the smear layer and the peritubular dentin. (J Prosthet Dent 1998;79:169-74.)