Triac, 3,5,3'-triiodothyroacetic acid, was administered at doses of 1, 3, 9 or 30 micrograms/100 g body weight to hypothyroid rats to determine its effects on TSH secretion and pituitary mRNA content. Triac caused a dose-dependent decrease in serum TSH 6 h after injection. Pituitary content of mRNA subunits either remained at hypothyroid levels or increased at 6 h. At 24 h after injection of the 3 micrograms dose of triac, serum TSH returned to hypothyroid levels; both alpha and beta mRNA subunits were reduced at this time. When 30 micrograms triac/100 g body weight was administered, serum TSH levels remained depressed 24 h later, while pituitary mRNA content was essentially the same as in the hypothyroid controls. These findings indicate that the initial decrease in TSH secretion in response to triac is independent of the availability of TSH mRNA transcripts and suggest that TSH secretion and synthesis may be differentially controlled.