Apexification with use of calcium hydroxide preparation was performed on immature teeth of young adult dogs and the apical bridges at the apex region were investigated radiologically and electron microscopically. The results were as follows : 1. Radiograms showed apical closure by a radioopaque apical bridge in the experimental group of 6 months. Microphotograms were also in agreement in the finding. 2. In measurements of photographic density, the apical bridge of the experimental group at 8 months showed similar values to those of apical cementum of the control. 3. Scanning electron microscopic findings showed the apical bridge to be cementum. The apical bridge was clearly separated from dentin on the root canal side by a crack line containing preparation residue. 4. Scanning electron microscopically, the lateral apical surface of the apical bridge at 6 months was rich in fibers and contained globuli, and the margin of cementum lacunae was surrounded by matrix fibrils. At 8 months, Sharpey fibers penetrated into the apical bridge, the margin of cementum lacunae was distinct, and cementum canaliculi also were observed. These findings were about the same as those of the cementum structure in the control. Accessory branch-like structures were observed in the section of the apical bridge. 5. Transmission electron microscopic findings for the control at both 6 and 8 months indicated many areas clearly showing lining cell layer of cementoblasts. Most of these cementoblasts presented the appearance of resting cells. In the experimental group, lining cell layers were indistinct and there were large cells with many cell processes. These active cementoblasts containing many mitochondria and r-ER were seen sporadically. These findings were remarkable at 6 months. 6. In the experimental group, it was suggested that active cementoblasts secreted vesicle-like structures, produced collagen fibrils, and possessed resorptive function of collagen fibrils. 7. Findings on cementocytes in the apical bridge were in good agreement with those reported by predecessors. Also, gap junction was observed between cell processes of cementocytes.