Quantitative data are presented to show incidence of spermatogonia and spermatocytes in normal mouse testicles at stated intervals in a 24-hour period and compared with data for testicles of mice killed at hourly intervals at 24-hours post-irradiation. Testicles were exposed to a single dose of 1440 r for 18 min. 40 sec. Radiation was obtained from a G. E. Maximar, 250 KvP X-ray unit. The following factors were used: 100 KvP, 15 Ma Hvl 2.6 mm. Al, 77r per min. at TSD of 32 cm. for 18 min. In comparison to normal testes, irradiated testes showed that active mitosis is inhibited for at least the first 24 hrs. and resting spermatogonia decrease in incidence after 20 hrs., thus confirming the belief that mitotic and resting spermatogonia are the most sensitive germinal cells. A prophase-appearing, spermatogonial-like cell did not decrease in incidence, thus suggesting that this cell is not a true spermatogonial cell.