The purpose of this study was to demonstrate changes in alveolar bone surrounding and blood vessels in the molar teeth of rats fed a high-protein low-calcium diet for various periods of time. Five-week-old male Sprague-Dawley rats were used. Control rats were given a normal diet; whereas the experimental rats were fed a high-protein low-calcium diet for 3, 5, 7, 10, 14, 18 or 21 days. Samples were observed by means of light microscopy and scanning electron microscopy, and the following results were obtained. 1. The changes appeared earlier in the dental alveolus than in the diaphysis of the femur. 2. Early changes in the alveolus were observed clearly in the 7-day experimental group. 3. Bone resorption progressed mesio-distally and cervically from spongy bone with the passage of time, and severe osteoporosis was observed in the 18-day experimental group. 4. An enlarged marrow cavity caused by bone resorption was observed, but the height and overall shape of the alveolar bone showed no changes, indicating the continuance of its support function. 5. The architecture of blood vessels in the bone marrow changed its form gradually from 7-day experimental diet and the vessels appeared to have been functioning like a cushion against mastication or physiological force acting on bone. 6. The vessels in the periodontal membrane exhibited characteristic changes in the cervical and apical resins of the membrane as bone resorption proceeded and changes in vessels in the bone marrow also progressed, and the vessels showed a relatively dense distribution there.