The growth of follicles in the immature mouse ovary was investigated in autoradiographs prepared after pulse-labelling with tritiated thymidine. Three parameters, which determine follicle growth were estimated: The number of follicles present in the ovary at different ages. The time it takes follicles to grow from one stage of development to another. This was calculated from the total number of granulosa cells in these stages and from their doubling times. The doubling time of granulosa cells was determined from their labelling index and the duration of their DNA-synthesis phase. The number of follicles, which start on their development at different ages. It was found, that the follicle development is not constant in the period from birth to maturity, but varies considerably. More follicles start to grow in the young than in the older immature mouse. Moreover the follicles grow faster early in life than later. The development from a follicle with one layer of granulosa cells to one with several layers and antrum formation takes about 10 days in the first half of the immature period, while it takes about 16 days as the animal approaches maturity. It was furthermore shown, that about 850 follicles start to grow in the immature period.