SUMMARY: Mucification of the vaginal epithelium in rodents requires the presence of both oestrogens and progesterone. The effect of different doses of hormones on the vaginal epithelium of spayed animals is briefly described. In spayed pregnant guinea-pigs maintained by progesterone, mucification of the vagina similar to that of pregnant controls develops by about days 18–20 post coitum, indicating a secretion of oestrogen by the developing placenta adequate to transform the vaginal epithelium, but probably of < 1 μg daily.