The ovary of the 30-d-old rabbit contains only small follicles with, at most, 3 or 4 layers of cells. We have estimated the labelling index of the follicular cells only from follicles having at least one labelled cell and using a zero truncated binomial distribution. The labelling index of the follicular cells of such follicles was weak and enlarged with the number of cells. It never exceeded 20%. Repeated injections of tritiated thymidine up to 8 times entailed a significant increase of the labelling index even for the smallest follicles. The labelling index of follicles was significantly increased 30 h after 1 injection of FSH (2 mg P-FSH). These results confirmed that the fraction of follicular cells that proliferate was low in the young rabbit ovary and the doubling time of the cells was large and that at this age, FSH increased the proliferation of cells.