Responses of odontoblasts and pulp capillaries to cavity preparation were investigated in the upper first molar teeth of rats, using light and transmission electron microscopy. At 100 days of age, the blood vessels of the pulp formed a subodontoblastic network consisting of continuous capillaries at a short distance from the odontoblast layer. Cavity preparation caused the displacement of some odontoblasts into the dentinal tubules, while others were separated from the predentin by rapid inflammatory exudation after drilling. The subodontoblastic capillary network under the injured dentin was shifted inwards together with the separated odontoblasts. The endothelium of the shifted capillaries showed a remarkable increase of pinocytotic vesicles, an event thought to be closely related to the formation of the exudative lesion. By one day after cavity preparation, most of the damaged odontoblasts had degenerated. Many cells with high nucleus/cytoplasm (N/C) ratios and prominent nucleoli accumulated around the subodontoblastic capillaries, some of which had many endothelial fenestrae facing these cells. These cells were suggestive of newly differentiating odontoblasts receiving nutritional supply from the capillaries. Three days after cavity preparation, newly differentiating odontoblasts took the place of the degenerated odontoblasts. They began to produce reparative dentin by five days after cavity preparation. Capillaries were located beneath the newly differentiating odontoblasts, but endothelial fenestrae gradually decreased in number. During the active reparative dentin formation, capillaries remained closely beneath the new odontoblast layer. Although the rate of reparative dentin deposition was not significantly lower than that in the primary dentin formation, one could not recognize an invasion of capillaries into the odontoblast layer nor a remarkable increase of endothelial fenestrae, both of which are common in active primary dentin formation. The results suggest that the function of capillaries differs between primary and reparative dentin formation.