The purpose of this study was to clarify that scintigram was a more effective method than radiogram in the early detection of periapical lesion. Periapical lesions were produced artificially by the opening of the pulp chambers of the lower right 3rd and 4th premolars in 6 dogs. The serial bone scintigrams using 99m-Tc-MDP and periapical radiograms were taken weekly. The uptake counts of the 99-Tc-MDP in the experimental side were compared with those in the control side. The periapical radiograms were interpreted with the joint evaluation by three dental radiologists. The following results were obtained; 1. The radioactivity in the experimental side was increased at the Ist week except one animal in which the radioactivity was increased at the 2nd week. 2. It was observed that increasing amount of radioactivity per week was prominent from the 1st day of experiment to the Ist week, and the 3rd week to the 4th week. 3. The radiographic evidence of the periapical lesions was observed at the 3rd week and became more apparent at the 4th week. 4. Histologically, proliferation of blood vessels and infiltration of chronic inflammatory cells were observed at the 1st week and osteoblasts were found after the 3rd week.