Using an in vitro assay with isolated rat nuclei, we have determined that thyroid hormone causes a 4-6-fold increase in the synthesis of mRNA coding for phosphoenolpyruvate carboxykinase. Proportional changes were seen in the steady-state cytosolic mRNA levels for phosphoenolpyruvate carboxykinase. Dibutyryladenosine cyclic 3',5'-monophosphate, which stimulates transcription of the phosphoenolpyruvate carboxykinase gene in normal rats, remained effective in hypo- or hyperthyroid animals. The effect of epinephrine on transcription of the gene for phosphoenolpyruvate carboxykinase appears to be modulated by thyroid hormone.