The effects of dentin primers on the sensitivity of dentin were examined by measurement of the repetitive firing observed when a class V cavity prepared in the dentin of the lower jaw of a rabbit was irritated by a blast of compressed air. It was possible to conclude that the combined application of an aqueous solution of 35% hydroxyethyl methacrylate and a commercially available dentin bonding agent was effective for reducing the repetitive firing and for maintaining such a "sedative " effect. Although the dentin tubules were sealed mechanically by the resin material, it was impossible to explain the mechanism by which the dentin primer application reduced firing.