Thyroid hormones exert profound effects on the energy metabolism. An inspection of the early and more recent literature shows that several targets at the cellular level have been identified. Since their effects on the nuclear signalling pathway have already been well-defined and extensively reviewed, this article focuses on the regulation of mitochondrial activity by thyroid hormones. Mitochondria, by virtue of their biochemical functions, are a natural candidate as a direct target for the calorigenic effects of thyroid hormones. To judge from results coming from various laboratories, it is quite conceivable that mitochondrial activities are regulated both directly and indirectly. Not only triiodo-L-thyronine, but also diiodothyronines are active in regulating the energy metabolism. They influence the resting metabolism in rats with 3,5-diiodo-L-thyronine seeming to show a clearer effect.