To elucidate the relationship between follicular growth and granulosa cell kinetics, ovarian histology was examined in mice treated with 5-bromo-2'-deoxyuridine (BrdU) or in combination with colchicine. In normal-cycling mice, active proliferation of the granulosa cells started in some follicles with diameters of more than 30 microns and all follicles more than 70 microns in diameter had already grown. Thus follicles between 30 and 70 microns were selected to start growing from those remaining in the resting phase. Although the granulosa cells synthesize DNA and mitosis took place throughout the epithelia in the follicles < 300 microns, such proliferating cells were distributed centripetally in follicles > 300 microns. In follicles measuring more than 400 microns in diameter DNA synthesis and cell division occurred in the inner epithelial layers after which, some of the divided cells moved to the outermost mural layer as fully differentiated cells to prepare for ovulation.