The sensitivity of experimentally exposed dentin, where the conditions at the outer tubule apertures were changed by various treatments, was studied in 40 contralateral pairs of teeth by intra-pair comparison. The results showed: (1) Dentin areas exposed by fracture were more sensitive than wet-ground dentin surfaces to application of dry, absorbent paper, scraping with a probe and a short blast of air. (2) Ground dentin surfaces treated for 2 minutes with 50% citric acid were more sensitive to the above mentioned stimuli and to application of a hyperosmotic solution (4-M CaCl2) than were untreated ground surfaces. (3) The sensitivity of ground dentin surfaces was markedly reduced by dehydration by a jet of air for 1 minute. The sensitivity rose after rehydration of the dehydrated surfaces by rubbing with water or gentle wet grinding. It is concluded that mechanical and dehydrating pain-producing stimuli are more readily transmitted from a dentin surface when the exposed tubule apertures are wide open, and the tubule contents are thereby easily accessible. This is the case in fractured and acid-treated surfaces, while following grinding and dehydration the apertures are blocked, and this results in a less sensitive surface.