The purpose of this study was to investigate whether the ovulatory cycle interferes with the effect of immobilization exposure on the expression of the type I CRF receptor (CRF-R) gene in the brains of female rats. The reproductive stages of adult female rats (200-250 g; 14 h of light; lights on at 0600 h) were verified by daily vaginal smears taken every morning for a minimum of three or four cycles before the experiment. Three hours after beginning the 90-min immobilization session, rats were deeply anesthetized and transcardially perfused with a solution of 4% paraformaldehyde in the morning (1100 h) and the afternoon (1700 h) of proestrus and diestrous day 2. Frozen brains were mounted on a microtome, cut in 30-microns slices, and processed for the detection of CRF-R messenger RNA (mRNA) by in situ hybridization histochemistry. Strand-specific antisense riboprobe was generated by in vitro transcription using T7 RNA polymerase and [35S]UTP. Basal levels of CRF-R transcripts were observed in several defined regions of the brain, such as the medial septal nucleus, nucleus of the diagonal band, basolateral and medial nuclei of the amygdala, red nucleus, pontine gray, and various layers of the cerebral cortex. Although control animals displayed almost undetectable levels of CRF-R mRNA in the paraventricular nucleus (PVN), immobilization stress induced a marked expression of the gene encoding CRF-R in the dorsomedial parvocellular division of this hypothalamic structure in immobilized female rats. The large majority of CRF-R-positive cells were colocalized in CRF-immunoreactive neurons. Interestingly, stress-induced CRF-R transcription was significantly higher in the morning of proestrus than in the afternoon and on diestrous day 2. Apart from the PVN, none of the other sites exhibiting basal signals of CRF-R mRNA in the brain was affected by acute exposure to immobilization. These results provide evidence that neurogenic stress can stimulate the transcription of the type I CRF-R selectively in the parvocellular PVN, a phenomenon influenced by the neuroendocrine events regulating the reproductive cyclicity in female rats.