It has been controversial whether thyrotropin-releasing hormone (TRH) may be involved in the regulation of biosynthesis of anterior pituitary thyroid-stimulating hormone (TSH), whereas TRH is well known to control the secretion of TSH from anterior pituitaries. We therefore studied the effect of the destruction of paraventricular nuclei (PVN), containing TRH neuronal cell bodies, on anterior pituitary TSH beta- and alpha-subunit mRNA levels the thyroidectomized rat. Median eminence TRH contents and serum TSH levels significantly decreased even 1 day after PVN destruction, whereas anterior pituitary TSH beta- and alpha-subunit mRNA levels did not change. In contrast, these mRNA levels significantly decreased by 3 days after PVN destruction. Growth hormone mRNA levels were not affected during 7 days after PVN destruction. Although the involvement of other factors related to the PVN is not totally excluded, these results raise the strong possibility that hypothalamic TRH regulates anterior pituitary TSH mRNA levels.