The purpose of this study was to assess the effect of citric acid and tetracycline HCl application to dentin surfaces by a "passive dripping" or an "active burnishing" technique. Twenty dentin blocks were prepared from freshly extracted non-diseased human impacted third molars. The blocks were root planed and randomly assigned to two groups for treatment with either citric acid or tetracycline HCl. The duration of treatment was 30, 60, 120, or 240 seconds. Control blocks were treated with distilled water. After treatment the blocks were processed for observation and measurements in the scanning electron microscope (SEM). Application of either of the acid solutions resulted in removal of the smear layer. Measurements indicated a time dependent increase in the mean dentinal tubule orifice diameter ranging from 1.05 microns in control specimens to 3.18 microns after 4 minutes treatment (citric acid group). The increase in tubule diameter was significantly greater (P less than or equal to 0.01) for both citric acid treatment modalities than tetracycline HCl treatment. There was also a time dependent increase in the depth of penetration as measured by a trumpeting of the tubule profiles, and this penetration was significantly greater (P less than or equal to 0.01) after citric acid treatments. Passive or active application of the acids did not seem to have any major impact on the measurements or on the surface morphology. It was concluded that citric acid causes more extensive changes than tetracycline HCl and that the mode of application of the agent is probably not critical.