Progesterone given at the same time as oestradiol-17\g=b\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.

The duration of stages of the cell cycle in the uterine epithelial and stromal tissues of ovariectomized mice was estimated by the labelled mitosis method. In untreated animals the mean duration of the S phase (DNA synthesis) was 10\m=.\5h in the glandular and luminal epithelium. Oestrogen treatment shortened it to 6 h in both tissues. In the endometrial stroma of progesterone\x=req-\ treated mice the duration of S was 8 h; when oestrogen was given it increased slightly. The generation times estimated under steady-state conditions were 270,156 and 383 h respectively in the lumen, glands and stroma of untreated mice. After oestrogen stimulation the responses became highly synchronized.

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Adult virgin, regularly cyclic female guinea-pigs were treated with different doses of luteinizing hormone (LH) and follicle-stimulating hormone during the late luteal phase of the oestrous cycle. The onset of the next oestrus was delayed and the length of the dioestrous stage was prolonged beyond the normal range in all the treated groups, but for a maximum period in the animals receiving LH alone.The results indicate a dual role of LH on the normal cycle of the guineapig. It has been suggested that either luteotrophin release from the pituitary or an intrinsic luteotrophic action is probably a part of the function of LH in addition to its ovulation-inducing function in the guinea-pig.