The antiprogesterone RU486 injected on proestrus to cyclic rats advances the preovulatory surge of LH, resulting in a 3-day estrous cycle. To ascertain whether proestrous progesterone secretion regulates ovulation by synchronizing the functions of the ovary, the pituitary and the hypothalamus, in this study the effects of RU486 (4 mg/0.2 ml oil/s.c.) at 09:00 h on proestrus (day 1), on LHRH secretion into the pituitary stalk blood vessels and on peripheral plasma concentrations of LH and 17beta-estradiol at 10:00 h and at 18:00 h on each day of the estrous cycle, have been investigated. Control rats present the expected surges of LHRH and LH at 18:00 h on day 1 and a second increase in LHRH but no LH secretion was found in the afternoon of day 2. RU486 decreases both LHRH and LH surges in the afternoon of day 1, while it increases plasma concentrations of LHRH and LH in the morning of day 2. During the rest of the estrous cycle RU486-injected rats show high basal secretion of LH in comparison to controls, and at 18:00 h of day 4 advanced preovulatory surges of both LHRH and LH are present in RU486-injected rats. In relation to 17beta-estradiol concentrations, RU486 injection increases those on day 3 and induces an advanced preovulatory surge of 17beta-estradiol in the morning of day 4. These results show that, in the 4-day cyclic rat, the 24 hours-shortening of the estrous cycle induced by the antiprogesterone RU486 is due to the enhanced secretion of estrogen during the days of metestrus and diestrus, which stimulates the release of a surge of LHRH in the afternoon of diestrus and this, in turn, induces the advanced preovulatory surge of LH.