The aspiration of odontoblasts into the dentinal tubules as an immediate effect of physical stimuli applied to exposed dentin seems to result from loss of sub­ stance at the distal apertures of the tubules and subsequent outward flow of the tubular contents through capillary action. A hydromechanical theory could account for the hitherto inexplicable sensitivity of the dentin to pain, in spite of the absence of nerve fibers in this tis­ sue. I f movement occurs rapidly enough, displacement of tubular contents may produce deformation of nerve fibers in the pulp or predentin or damage to odon­ toblastic cells. Both these effects may produce pain.