During the ovarian cycle the mouse ovary undergoes changes which involve considerable movements of cells. Autoradiographs prepared at different time intervals after flash labelling with 3H-thymidine show that it is possible to mark cells in distinct cell groups in the ovary and to follow their growth, movement and disappearance from cycle to cycle for a considerable period of time. This method has been used to follow follicle development, the movements of corpora lutea and the development of the peripheral stroma through five cycles.