The isolated perfused rat kidney was used to study renal handling of thyroid hormones. Substrate enriched protein free perfusates were chosen which allowed the kidneys to perform a normal glomerular filtration rate of 1.1 ml/g kidney-min and enabled accurate estimation of filtered load and tubular handling of free thyroid hormones without interference from the presence of hormone binding substances. Under these conditions we found a high tubular reabsorption capacity for both T 3 and T4. This reabsorption capacity was unsaturable, even when the filtered hormone load was far above (x 100) the physiological range.