We have attempted to ascertain the proportion of the rat hepatic genome that is under the selective influence of thyroid hormones and to describe the response patterns of individual mRNA sequences in the transition between hypothyroidism and euthyroidism and between euthyroidism and hyperthyroidism. Poly(A)+RNA was extracted from livers of thyroidectomized, intact, euthyroid rats and of thyroidectomized rats rendered euthyroid and hyperthyroid with daily doses of triiodothyronine. The extracted RNA was translated in a reticulocyte lysate system in the presence of [35S]methionine, and the products were analyzed by two-dimensional gel electrophoresis. Triiodothyronine attenuates as well as augments the expression of certain genes at a pretranslational level. This could represent either a direct or an indirect action of the hormone. Triiodothyronine influences approximately 8% of the 231 mRNA sequences visualized, stimulating activity in 11 and inhibiting activity in 7 sequences. Translational activity of at least one mRNA sequence decreased in both thyroidectomized and hyperthyroid animals, compared to euthyroid levels. The relationship of mRNA response to receptor occupancy varied with examples of linear and amplified responses and responses that were maximal at less than full nuclear occupancy.