Normal puberty onset in female rats treated with antiprogesterone RU486 during juvenile period.

The significance of the increased progesterone plasma levels after day 21 on the onset of puberty in female rats is unclear. To analyze this question, female rats were injected from day 21 to 32 with the potent antiprogestagen RU486 (1 mg/day) or vehicle. Treatment with RU486 did not modify either the age at which vaginal opening or first estrus occurs. Vaginal cycles were regular. Animals sacrificed on day 40 or 50 showed reduced uterus weight, whereas body, pituitary, adrenal and ovaries were not affected. These results evidence that antagonization of progesterone action during the juvenile phase did not affect the onset of puberty.