Microinjection of progesterone into the preoptic area (POA) of the rat on either day 2 of dioestrus or the morning of pro-oestrus inhibited the ovulatory peak of LH which would normally appear during the afternoon of pro-oestrus. However, when progesterone plus oestrogen were used, the inhibition of the surge of LH was observed only if the microinjection was applied on the morning of day 2 of dioestrus. Microinjection of progesterone into the medial basal hypothalamus (MBH) at day 2 of dioestrus advanced the ovulatory peak by 24 h. This effect was also observed when progesterone and oestrogen were simultaneously infused. Progesterone injected into the MBH on the morning of pro-oestrus significantly increased the release of LH during the afternoon. This effect was also observed when progesterone plus oestrogen were simultaneously infused. Application of progesterone alone or with oestrogen to the nucleus dorsalis raphe on the morning of either day 2 of dioestrus or pro-oestrus induced a significant decrease of the levels of LH in plasma and prevented the appearance of the ovulatory peak during the afternoon of pro-oestrus. These findings indicated that these two steroids have differential actions when infused into various hypothalamic and extrahypothalamic nuclei. Consequently, it is concluded that in studies on the effects of ovarian hormones it is important to take into account not only the isolated action of each one of them, but also their interaction on the effects of the diverse neural structures involved in the control of ovulation on each day of the ovulatory cycle.