Calcium hydroxide paste was applied to human dentin in vitro to determine its effects on dentin permeability. Discs of dentin were acid-etched on both sides to permit determination of their maximum permeability. Smear layers were then applied to the enamel sides of the discs, thereby reducing dentin permeability 99%. Topical application of Ca(OH)2 paste to the smear layer reduced dentin permeability further, to levels 48% below that of untreated smear layers. When the Ca(OH)2-treated smear layers were exposed to 6% citric acid for two min, dentin permeability returned to the initial acid-etched value, demonstrating that Ca(OH)2 offers little protection to acid challenge. Treatment of acid-etched dentin with Ca(OH)2 produced a similar reduction in dentin permeability, which was restored to normal following acid challenge. Thus, Ca(OH)2 is effective at reducing the permeability of both the smear layer and of acid-etched dentin, in vitro.