The hypothalamopituitary-adrenal axis exhibits a diurnal rhythm as witnessed by the daily excursion of corticosterone in plasma. The rhythm appears to be mediated largely by the stimulation of CRH neurons in the paraventricular nucleus (PVN) of the hypothalamus. In the present study, we investigated the effects of circadian influence on CRH mRNA levels in the paraventricular hypothalamus. Animals were sacrificed through a 24-hour period to establish a detailed time course of CRH mRNA fluctuations. Levels of both type I and type II corticosterone receptor mRNAs were also measured in this area to see whether changes correlate with that of CRH mRNA. Plasma levels of ACTH were quantified as an index for CRH peptide secretion. The results indicate that changes in ACTH closely paralleled alterations in corticosterone levels with an increasing trend starting at 1 PM, suggesting that the diurnal secretory drive commences around this time. The CRH mRNA rhythm as determined by RNase protection assays appeared to change in an anticipatory fashion to these endocrine fluctuations, increasing during the light phase and reaching maximal levels just prior to dark (5-6 PM). An abrupt decrease of 30% in the CRH mRNA content was detected in the hypothalamus within 2 h after dark (8 PM) and coincided with the peak of plasma corticosterone levels. However, other periodic variations in the CRH mRNA content were not accompanied by changes in plasma corticosterone. Neither types of corticosterone receptor mRNAs showed any diurnal change suggesting that the expression of steroid receptors in the hypothalamus is not regulated by circadian influences. We conclude that CRH mRNA levels fluctuate diurnally but are inversely related to corticosterone levels only in the early evening.