The authors utilized two groups of adult male rats to study the effect of chronic-uremic toxemia on the synthesis of thyroid hormones. The control group was sham operated, while the other group had about 70--80% of the left kidney tissue surgically excised and a total right nephrectomy performed 10 days later. At the 90th day after the second operation, 8 muCi of 131I were injected intraperitoneally and 24 h later the thyroid was excised and hydrolyzed enzymatically for a period of 14 h at 37 degrees C. The percentages of iodinated components were then calculated after radiochromatographic separation on paper. The group of experimental rats by this time showed significant increases of plasma urea and creatinine, and an absence of acidosis. The results suggest that monoidotyrosine concentration is increased and diiodothyronine decreased in the experimental group of rats with renal failure. This might indicate a toxic metabolic blockage in the transformation of monoiodotyrosine to diiodothyronine.