We showed previously that the level of TRH receptor (TRH-R) mRNA in rat pituitary GH3 cells is down-regulated by TRH. Here, we study the mechanism of regulation of TRH-R mRNA in a line of GH3 cells that are stably transfected with mouse pituitary TRH-R cDNA (GH-mTRHR-1 cells). GH-mTRHR-1 cells were found to have 2.4 times the number of TRH-Rs and to stimulate a 2.5-fold greater increase in inositol phosphates in response to TRH than the parent cell line and to show TRH-induced down-regulation of TRH-R number. GH-mTRHR-1 cells contained 26 +/- 1.6 molecules of mouse TRH-R mRNA/cell and 230 +/- 31 molecules of mRNA for the neomycin resistance gene (NEO) with which it was cotransfected. In GH-mTRHR-1 cells, TRH caused a dose-dependent transient decrease in mouse TRH-R mRNA, with a nadir to 20% of control levels after 6 h. In contrast, TRH did not affect NEO mRNA or glyceraldehyde phosphate dehydrogenase (GAPDH) mRNA, an endogenous gene product. TRH stimulated the rate of transcription of mouse TRH-R DNA by approximately 2-fold, but did not affect total poly(A) RNA synthesis. Most importantly, TRH caused a 4-fold increase in the rate of degradation of mouse TRH-R mRNA, but did not affect degradation of GAPDH mRNA. The half-lives of mouse TRH-R and GAPDH mRNAs were 3 and more than 20 h in control cells and 0.75 and more than 20 h in cells treated with 1 microM TRH for 1.5 h, respectively. These data show that the predominant effect of TRH on mouse TRH-R mRNA in GH-mTRHR-1 cells is to enhance the rate of its degradation. We suggest, therefore, that down-regulation of TRH-R mRNA caused by TRH in the parent GH3 cell line is secondary to increased TRH-R mRNA degradation.