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The purpose of the present study was to investigate the effect of a single dose of RU 486 administered very early in the secretory phase on endometrial development and levels of progesterone receptors, on plasma levels of gonadotrophins and ovarian hormones and on the pattern of menstrual bleeding. Twenty-four regularly menstruating subjects participated and were studied during a control, a treatment and a follow-up cycle. In the treatment cycle, a single dose of 200 mg RU 486 was given in the evening of the second day after the urinary LH peak. Plasma was collected from cycle day 10 until menstruation in both control and treatment cycles. The lengths of the control, treatment and follow-up cycles were equal. Three of the subjects had slight vaginal bleeding in association with RU 486 intake which, however, did not disturb their normal menstrual rhythm. Plasma levels of oestradiol, progesterone and FSH were not affected in the treatment cycle, whereas LH levels increased slightly. The elimination half-life of RU 486 was 28.6 h. An endometrial biopsy was taken 12, 36 or 84 h (LH + 3, LH + 4 and LH + 6) after drug intake (eight subjects in each group) and another biopsy was taken on the corresponding day in the control cycle. The specimens were assessed by morphometric analysis and for cytosolic progesterone receptor concentrations. Endometrial biopsies taken 12 h (on LH + 3) after RU 486 intake contained significantly (P less than 0.001) lower levels of cytosolic progesterone receptors than in the control cycle, but levels at 36 and 84 h were similar.(ABSTRACT TRUNCATED AT 250 WORDS)

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