Abstract The generation time of the duodenal crypl cells of BCF 1 mice exposed to 12 r per day gamma irradiation for 200 days starting at 100 days of age was compared to that of 300-day-old nonirradiated controls using tritiated thymidine. It was found that the generation time was approximately 12 hr in the irradiated group compared with approximately 15 hr in the control group. The results suggest that the difference in generation times is due to differences in the duration of S and or G 1 stages. The number of crypt cells initially labeled after the injection of tritiated thymidine was less by about 2 cells per crypt in the irradiated mice. The interanimal variation was less in the irradiated group and was similar to that found in young CAF 1, mice (93 days of age).