The content of mRNA coding for the precursor of CRH in the hypothalamic paraventricular nucleus (PVH) was measured in intact male and estrogen-treated ovariectomized female rats at different times of the day using in situ hybridization histochemistry. Plasma corticosterone concentrations were measured in separate groups of animals at the same time points, with females showing higher levels than males at all times, and both sexes exhibiting the well characterized diurnal rhythm. There was a significant decrease (greater in females) in the content of prepro-CRH (ppCRH) mRNA between midday and midnight in rats of both sexes. Previous studies have demonstrated that the content of ppCRH mRNA in the PVH responds inversely to changes in the concentration of plasma corticosteroid in a concentration-dependent manner. The present study suggests that ppCRH mRNA in the PVH may respond in a similar way to fluctuating concentrations of plasma corticosterone throughout the day, and that although the exact mechanism remains to be elucidated, diurnal changes in the rate of CRH synthesis may be involved in the expression of diurnal rhythms within the hypothalamo-hypophysial-adrenal axis.