The excretion of FSH, LH, oestrogens and pregnanediol was monitored once weekly in urine samples collected from eight peri-menopausal women aged 44-55 years (median, 52 years). Observations were commenced between 5 and 15 weeks before the menopause and were continued for 22-30 weeks after final menstruation. Amenorrhoea of greater than 2 years duration in association with a persistent elevation in gonadotrophin output was considered proof of the post-menopausal state. No clear hormonal change occurred at the time of the menopause. During the peri-menopause there is a transition from the regular ovulatory cycles of pre-menopausal women to the unvarying high gonadotrophin and low oestrogen excretion which is generally regarded as being characteristic of post-menopausal women. In the group studied, post-menopausal levels of FSH and LH were common before and episodes of high oestrogen excretion were not uncommon after final menstruation. Menstrual failure appeared to occur spontaneously at some stage during the transition from the pre- to the post-menopausal state, and not to be associated with its conclusion. From the hormonal point of view the immediately post-menopausal period could not be distinguished from the long cycles of peri-menopausal women. This suggests that an endometrial rather than a hormonal event might determine the time at which menstruation stops during the menopausal transition.