The thyroid function was explored by comparing serum total and free iodothyronine levels in young male genetically obese Zucker rats and in their lean littermates, aged from 6 to 8 weeks old. Total and free thyroxine (T 4) and 3, 5, 3′ triiodothyronine (T 3) levels were significantly decreased in obese rat serum while total 3, 3', 5'-triiodothyronine (rT 3) remained constant. Radioactive T 4 half life is slower in the plasma of obese rats. Peripheral synthesis of T 3 from deiodination of T 4 is also decreased in obese rat liver homogenate. These modifications produce changes in liver mitochondria oxidative phosphorylation and in marker enzyme activity, which are usually associated with hypothyroidism and hypothalamic disturbances.