The variations in size, numbers and growth rates of mouse ovarian follicles were investigated at different stages of the oestrous cycle. Autoradiographs were prepared of sections of ovaries after pulse-labelling with tritiated thymidine. By determining the growth rate of the granulosa cells expressed as their doubling times, it was possible to estimate the exact growth rates of whole follicles. The number of follicles which began developing during a particular time interval was also determined. The results show that the number of large follicles fluctuates with the cycle, and that the size of the largest follicles increases during the cycle. Follicles of medium size grow faster at oestrus that at any other time. This is in contrast to the large follicles, in which only minor variations in growth rate were noted. It was moreover shown, that more follicles begin to grow at oestrus than at other periods during the cycle. It was concluded, that the reduction in the number of small follicles with age is mainly due to follicle development rather than to the degeneration of small follicles. The time required for the full development of an ovarian follicle is 19 days.