SUMMARY During the day of pro-oestrus in rats showing regular 4-day oestrous cycles, the concentration of progesterone in peripheral plasma increased slowly from 09.30 to 13.30 h and more rapidly during and after the 'critical period' for luteinizing hormone (LH) release to reach a level of 24·4 ng/ml at 21.00 h. Administration of sodium pentobarbitone at 07.00 or 13.30 h (but not at 16.30) on the day of pro-oestrus delayed ovulation and also prevented the rise in progesterone concentration at 21.00 h on that day. Levels were found to be high 24 h later on the evening of the day preceding the delayed ovulation. It is concluded that the major rise during and after the 'critical period' is the result of LH stimulation of ovarian progesterone secretion. The possibility that the gradual increase in peripheral plasma progesterone concentration that occurred before the 'critical period' was due to adrenal secretion and played some role in facilitating the onset of the ovulatory surge of LH was examined. The rise in plasma corticosterone concentration roughly paralleled that of progesterone up to the 'critical period' but the curves for the two steroids later became divergent. Suppression of adrenal activity during pro-oestrus by the administration of dexamethasone phosphate resulted in a blockade of ovulation which could be reversed by the administration of either progesterone or corticotrophin (ACTH). Conversely, blockade of ovulation followed ACTH administration at metoestrus and approximately 50% of animals adrenalectomized or sham-adrenalectomized at metoestrus or dioestrus failed to ovulate at the expected time. In adrenalectomized rats tested 12–15 days after operation, the period of pro-oestrus during which sodium pentobarbitone administration could block ovulation was more prolonged than in intact rats. The possible roles of progesterone of adrenal origin in facilitating LH release and entraining the LH release mechanism to the light—dark rhythm and of progesterone of ovarian origin in ensuring full sexual receptivity are discussed.

The concentrations of androstenedione, testosterone, oestrone and oestradiol-17beta were measured in peripheral and ovarian venous blood and follicular fluid of women at various stages of the menstrual cycle. The concentration of oestradiol was similar in small follicles (diameter less than 8 mm) at all stages of the menstrual cycle and in large follicles (diameter greater than or equal to 8 mm) except during the mid- and late follicular phase when the concentration reached a peak (approximately 1500 ng/ml). The concentration of androstenedione was lowest in large preovulatory follicles at midcycle at a time when the secretion into the ovarian vein was markedly increased. The concentration of testosterone in large follicles (greater than or equal to 8 mm) was unchanged during the follicular phase whereas in small follicles there was a peak at mid-cycle. The rise in the concentration of testosterone and androstenedione at mid-cycle in peripheral plasma may be due to increased secretion by the preovulatory follicle into the ovarian vein. It is suggested that the relatively low concentration of androstenedione in follicular fluid of the preovulatory follicle arises from increased aromatization by granulosa cells in the course of oestrogen synthesis.

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Summary Total plasma oestriol in pregnancy was estimated by a competitive protein binding method. The advantages of the measurement of plasma oestriol levels over the 24‐ or 48‐hour urinary oestriol assays are discussed. The pattern of plasma oestriol levels in the third trimester of pregnancy was investigated in over 400 healthy women and in general a uniform increase was found from the 28th week until term. Diurnal variation was found to be present, but this did not preclude the use of the method for assessing the feto‐placental unit. Exercise and meals had a minimal effect on the oestriol levels. Following delivery the level decreased sharply to reach, within four hours, one‐half that found just prior to delivery.