a-Hydroxy-pregn-4-en-3-one (20-OH-P), a steroid hormone secreted by the ovarian inter- stitium in rabbits, induced the appearance of ciliated cells in the endometrium of ovariectomized rabbits. Estradiol-1 7� appeared to inhibit this effect partially, but 20-OH-P and progesterone were antagonists, since their simultaneous administration did not result in either the formation of ciliated cells or a fully developed secretory endometrium. Withdrawal of 20-OH-P for 5 days, after its administration, resulted in a very rapid proliferative effect leading to the formation of arborized secretory endometrium in 48 h. Ciliated cells did not seem to be involved in this proliferative reaction, since the evidence presented here suggests that they are probably terminally differentiated cells. Furthermore, cytophotometric determination of DNA in individual nuclei showed that while ciliated cells are a homogeneous population of non- dividing cells arrested in G1 , the rest of the luminal epithelium is a heterogeneous cell population showing a bimodal DNA distribution with one peak in the G1 and the other in the G2 range. This last peak, not observed in either ovariectomized or intact rabbits, and very prominent in rabbits injected with 20-OH-P, probably represents a population of cells arrested in the G2 phase of the cell cycle. These cells, after the withdrawal of the 20-OH-P, probably reenter the cell cycle, playing a role in the formation of the arborized secretory epithelium.