Pregnant mothers were irradiated by a single dose of gamma rays (٠،٢،٤،٦ Gy, Cobalt ٦٠) in the days ١٠،١٢،١٤،١٦،١٨ of pregnancy. The heads of the embryos’, and those of the neonates were taken at consecutive intervals of irradiation, starting from ١٦ days of pregnancy till ٣rd day after delivery. The effect of irradiation was investigated in the development of the ٢nd and ٣rd lower molars on serial tissue sections, within consecutive periods of their organogenesis. Irradiation led to growth-deficiency in the ٢nd & ٣rd molars, and caused delay in their development. This was observed in various degrees depending on the dose, time of irradiation, and time after irradiation.