This study was designed to elucidate the distribution of the immunoglobulins in the experimentally induced rat periapical lesions. The pulp exposure was performed in 80 molars from 40 rats and the animals were sacrificed at 15, 30, 60 and 90 days after the operation and examined and radiographed. Of the 80 samples, 56 samples were routinely sectioned ($4-6{mu}$ in thickness) and stained with Hematoxylin-Eosin for the light microscopic examination and 50 samples were stained with toluidin blue for mast cells and 50 samples were stained using the Avidin-Biotin horseradish peroxidase for detecting the presence of Ig A, Ig E, Ig M and Ig G containing cells. The following results were obtained : 1. The periapical lesions could be observed in all of 80 teeth by radiogragh (100%) and the periapical lesions were detected in 50 samples of 51 samples by light microscopy (98%). The size of lesions increased with time lapse both by radiograph and by light microscopy(p<0.05). 2. Of the 50 samples, 19 samples were diagnosed as periapical abscesses, 18 as periapical granulomas, 10 as fibrous scar tissues and 3 cysts. 3. After pulp exposure, periapical granulomas were developed mostly in the 15 day group, with time lapse periapical abscesses and fibrous scar tissues increased. 4. In the 50 periapical lesions, the numbers of Ig G containing cell (57.2%) were prominent and the percentage of Ig A, Ig E and Ig M containing cells were 16.4%, 14.7% and 11.8% respectively. The numbers of all classes of immunoglobulin containing cell were highest in the periapical granulomas and lowest in the cysts(p<0.05). 5. The number of the mast cell and immunoglobulin containing cells decreased generally with time lapse after the pulp exposure and Ig A, Ig E, Ig M and Ig G containing cells and mast cells had the high correlation one another(>0.6).