SUMMARY The distribution of cell division in the tissues of the mouse uterus changes in a characteristic manner during the first few days of pregnancy due to changes in the secretion of ovarian hormones. Pregnant mice were ovariectomized at various times and attempts made to reproduce the pattern of cell division with exogenous hormones. The schedule of hormone injections producing a pattern of uterine mitoses which most closely approached that of pregnancy suggested that secretion of both oestrogen and progesterone started about 48 hours after mating and continued, possibly increasing, until after implantation had started. This conflicts with the oestrogen surge hypothesis. Further experiments were carried out to determine the optimum hormone treatment schedules for sensitivity to induction of the oil decidual cell reaction. Multiple small doses of oestradiol allowed the production of greater deciduomas than a single dose, thus again suggesting that the oestrogen surge hypothesis may not be correct.