A response of growth hormone (GH) to thyrotropin-releasing hormone (TRH) is observed in lower mammals and patients with diseases such as a cromegaly, but not in normal subjects. We have previously demonstrated the existence of intact TRH receptor mR N A in GH-secreting a denoma. To examine whether intact somatotrophs in the anterior pituitary also express TRH receptor, we attempted to localize both TRHR mRNA and GH immunoreactivity simultaneously. In situ hybridization analysis revealed TRHR mRNAs specifically in the anterior pituitary, and 61.1% of the anterior pituitary cells expressed this transcript. Staining for GH and PRL on the same samples showed that the somatotrophs apparently expressed TRHR mRNA and approximately 62.3% and 30.9% of hybridization-positive cells were somatotorophs and mammotrophs, respectively. Moreover, TRHR mRNA level in the somatotrophs expressed as the number of silver grains per cell was equivalent to that in the mammotrophs. These findings demonstrated expression of the TRHR mRNA in somatotrophs in the rat anterior pituitary, and also showed that more than 50% of the TRHR mRNA detected in the anterior pituitary was derived from these cells.