Thyroid hormone (T $ )-dependent gene expression of the adenine nucleotide translocase (ANT) and the FAD-linked glycerol 3-phosphate dehydrogenase (mGPDH) was investigated in several rat tissues. Both proteins provide an important link between cytosolic and mitochondrial metabolic pathways and seem to be involved in the stimulation of mitochondrial oxygen consumption in response to T $ . Here we show that two ANT isoforms are expressed in rat, the muscle-specific ANT1 form and the ubiquitous ANT2 form. The expression of ANT1 mRNA is not sensitive to T $ whereas the amount of ANT2 mRNA is increased 7-9-fold in liver and heart within 12-48 h after T $ application. Little or no effect of T $ on ANT2 mRNA was observed in kidney and brain. The mRNA changes are paralleled by an increase in ANT protein, thus explaining the accelerated ADP\ATP exchange observed in mitochondria isolated from hyperthyroid rats. The key role of ANT2 in the control of hyperthyroid