Endodontic preparations may induce changes in the permeability of radicular dentin. The aim of this study was to analyze the variation of hydraulic conductance measured in situ after three endodontic preparations (manual K+H files, ultrasonic preparation, and manual with NaOCI and EDTA). Scanning electron microscopic analysis of the canal walls was made to permit correlations between variations of permeability and the amount of surface covered with smear layer or organic debris. There was an inverse relationship between the variations in dentin permeability and the presence of smear layer. The significance of this correlation was variable according to the techniques used. Dentin thickness was as significant a factor in influencing radicular permeability as was the smear layer. The use of EDTA induced a considerable increase in radicular permeability. The use of ultrasonics produced a similar but weaker effect.