The effect of Sensodyne dentifrice, which contains strontium chloride hexahydrate, on the occlusion of dentinal tubules was studied in vivo and in vitro. Eight premolar teeth scheduled for extraction for orthodontic purposes were used. Four of the teeth were extracted before Sensodyne application (in vitro). The other four teeth were treated in vivo and then extracted. A patch of enamel was removed from all the teeth to expose the dentinal surfaces, and the smear layer over the exposed area was removed. In the in vivo test, the patients were instructed to brush their teeth for 1 minute, twice a day, for 2 weeks. The teeth were their extracted and kept in distilled water until examined by scanning electron microscope. The in vitro teeth were brushed in the same manner and kept in distilled water between brushings. Most of the in vitro dentinal tubules were found to be open, whereas most of the in vivo dentinal tubules were occluded.