The effect of werum on the uptake of thyroid hormones by the perfused rat heart.

1. The effect of serum on the uptake of radioactive L-thyroxine and L-3,5,3'-tri-iodothyronine has been studied in the perfused rat heart.2. With thyroxine binding protein (rat serum) in the perfusion fluid the uptake of both hormones is much reduced.3. This inhibitory effect of serum on hormone uptake is greater with thyroxine than with tri-iodothyronine.4. Serum in the perfusion fluid accelerates the release of accumulated thyroid hormone from the rat heart.5. High concentrations of thyroxine increased the thyroxine space of hearts perfused by solutions containing serum.