The effects of 1, 3 and 10 days of exposure to 12 R of continuous 60 Co γ-irradiation per day on the duration of the generation time (GT) and on the length of the four phases of the <tex-math>$\text{cycle}-{\rm G}_{1}$</tex-math>, S, G 2 , and M-in the crypts of the mouse small intestine were studied in 100-day, 400-day, and 825-day <tex-math>${\rm BCF}_{1}$</tex-math> hybrid mice. Estimates of the time spent in the total GT and its four phases by the percent labeled mitotic figure method following the labeling of those in S were made with <tex-math>${}^{3}{\rm HTdR}$</tex-math>. These estimates show that exposure to 12 R of 60 Co γ-rays per day for 1 to 10 days reduces the generation time of the intestinal crypt cells in all three age groups. The reduction is accomplished in less than 3 days in 100-day and 400-day animals but may require a longer interval to reach its maximum in the 825-day mice. The reduction in GT is greatest in the old mice, but this is primarily due to the longer preirradiation generation cycle. The GT lengthens between 3 and 10 days in the two younger groups but remains the same in the older animals. All the GT changes are related to changes in G 1 in all three age groups. The very early reduction in GT in all three age groups of mice after only 3 days of exposure to 12 R/day eliminates the selection hypothesis.