The effects of various ovarian hormones administered on the morning of pro-oestrus on gonadotrophin levels and the incorporation of 35S from methionine into protein of discrete areas of the brain and the anterior pituitary were investigated at 15.00 h of the same day in female rats. The hormones which were investigated in this study could be divided in general into two groups according to their actions. The first group, consisting of oestradiol-17 beta and progesterone, tended to advance the pre-ovulatory surge of luteinizing hormone (LH) by 3-6 h from 18.00-21.00 h, together with the peaks of [35S]incorporation in the median eminence area and the anterior pituitary which normally accompany the LH surge. The second group, consisting of the LH-stimulated reduced progesterone metabolites, 5 alpha-pregnane-3,20-dione (pregnanedione) and 20 alpha-hydroxy-pregn-4-en-3-one (dihydroprogesterone), tended to inhibit serum gonadotrophin levels as well as inhibiting the pro-oestrous increase of [35S]incorporation in the median eminence area and in the amygdala, but not in the preoptic area and the anterior pituitary. On the afternoon of pro-oestrus in intact animals, luteinizing hormone-releasing hormone or LH administration had the same effect on [35Si1incorporation in the brain as did the progesterone metabolites, though this effect was not observed if the animals had been ovariectomized a few hours beforehand. It is suggested that certain ovarian hormones are involved in the neural events which induce the pre-ovulatory LH surge, while others are associated with neural events which terminate the stimulus for the LH surge.