Technical Abstract: Physiological effects of thyroid hormones are mediated primarily by binding of triiodothyronine, to specific nuclear receptors. It has been hypothesized that organ-specific changes in production of triiodothyronine from its prohormone, thyroxine, target the action of thyroid hormones to the mammary gland and play a role in mediating or augmenting a galactopoietic response to bST. Additionally, tissue responsiveness to thyroid hormones may be altered by changes in the number or affinity of nuclear receptors for thyroid hormones. In the present study, effects of bST and bovine growth hormone releasing factor on thyroid hormone receptors in liver and mammary gland were studied. Lactating Holstein cows received continuous infusions of bST, or bovine growth hormone releasing factor (bGRF), or served as uninfused controls. After 63 d of treatment, nuclei were isolated and incubated with [125I]-triiodothyronine. Treatments did not alter the capacity or affinity of specific binding sites for triiodothyronine in liver or mammary nuclei. Evaluation of transcript abundance for thyroid hormone receptors showed that isoforms of thyroid hormone receptor or retinoid receptor (which may influence thyroid receptor action) expressed in mammary gland were not altered by bST or bGRF treatment. Data do not support the hypothesis that administration of bST or bGRF alters sensitivity of mammary tissue by altering the expression of thyroid hormone receptors.