This study investigated the expression of corticotropin releasing hormone (CRH) and its receptor CRHR-1, and arginine vasopressin (AVP) mRNAs during the stress hyporesponsive periods of late pregnancy and lactation (day-3) and in virgin stress-responsive females. In situ hybridization histochemistry showed that basal CRH mRNA in the paraventricular nucleus (PVN) decreased in pregnant and increased in lactating rats (compared with virgin controls), whereas it increased after restraint stress only in virgin rats. Basal PVN CRHR-1 mRNA increased markedly in all groups but reached lower levels in pregnant rats. Basal AVP mRNA in the parvocellular PVN was higher in lactating rats, and in contrast to CRH mRNA, it increased after stress in all groups. In medial preoptic area (MPOA) CRH mRNA levels were higher in lactating females compared with virgin and pregnant rats, and unexpectedly they decreased markedly after stress only in virgin rats. CRH mRNA levels in the central and medial nuclei of the amygdala were higher in lactating rats than in virgin or pregnant ones, and stress had no effect in either group. These data suggest that these stress hyporesponsive periods: (1) do not depend on basal CRH mRNA expression in the PVN; (2) appear to have intact stress-activated afferent pathways to the PVN, as shown by preservation of CRHR-1 and AVP responses to stress, but the information may be differently processed; (3) are associated with an alteration in a CRH mediated pathway from the MPOA.