The hydrodynamic theory of dentin sensitivity is based on the premise that sensitive dentin is permeable throughout the length of the tubules. Such a condition may permit the diffusion of bacterial products across dentin to the pulp where they may cause irritation of pulpal soft tissues. However, the slow outward movement of dentinal fluid tends to flush the tubules free of exogenous substances. This balance between the inward diffusion of exogenous substances, whether bacterial or the active ingredients in desensitizing medicaments, and the cleansing action of dentinal fluid flow needs to be examined experimentally.