The purpose of this study was to investigate the radiation effect on the stages of amelogenesis. Twenty ll-day-old rats which were irradiated by 4Gy of garmna radiation on the 19th prenatal day were used for the experimental group and twenty ll-day-old rats which were not irradiated were used for the control group. The length of each zone of amelogenesis were measured on the sagittal section using a light microscopic enlargement at 400 x the normal view while the morphologic changes of ameloblasts of each zone were observed electronmicroscopically. The obtained results were as followed : 1. The length of the region of facing pulp and facing dentin of the zone of presecretion were increased by 11.5％(P<0.05) and 17.7％(P<0.01), respectively. 2. The length of the zone of secretion was increased by 17.3％(P<0.01), but the zone of maturation was decreased by 15.3％(P<0.01). 3. The total length of the zone of amelogenesis was not changed significantly(P>0.05). 4. Electron-microscopically, enlargement of the cell membrane, rER, mitochondria, and nuclear membrane were observed. These changes were mostly severe in the zone of maturation.
