Progesterone given at the same time as oestradiol-17β prevented approximately 40% of uterine luminal epithelial cells from entering DNA synthesis, but did not prolong the G1, S or G2 phases in the remainder. However, it blocked the entry of these cells into a second round of DNA synthesis. It is suggested that uterine luminal epithelial cells are only sensitive to progesterone for a limited period in G1.