We investigated changes during the estrous cycle in cellular levels of corticotropin-releasing hormone (CRH) mRNA in parvocellular neurons of the hypothalamic paraventricular nucleus, using in situ hybridization. Intact female rats with 4 day cycles were sacrificed at 11 different times during the cycle at 09.00 h and 16.00 h on each day, with additional collection times at 14.00 h, 18.00 h, and 20.00 h on the day of proestrus. Twelve microns coronal sections of fresh-frozen brains were made through the paraventricular nuclei (PVN) and placed on gelatin-coated slides. A 48 base oligodeoxynucleotide probe complementary to the coding region for rat CRH was used to measure CRH mRNA. There was a sharp increase (P less than 0.01) in CRH mRNA in the ventral PVN between 14.00 and 16.00 h on the day of proestrus, at the approximate time of the ovulatory surge. Following this rise, there was an even larger decline (P less than 0.01) between P 16.00 h and P 20.00 h. Levels of CRH mRNA did not change greatly on other days of the cycle, nor were there significant changes in the dorsal PVN. Given the known effects of CRH on GnRH secretion, these changes occur at a time when they could serve to modulate the midcycle luteinizing hormone (LH) surge.