I ncreased metabolism of thyroid hormones was observed in rats adapted to an ambient temperature of 4°C. The increased hormonal degradation was manifested in enhanced metabolic, urinary deiodinative, biliary, and fecal clearances of iodothyronines. Increased metabolic clearances were due to stimulation of cellular hormonal disposition, evidenced by elevated intrinsic cellular clearances. After adaptation, the concentration of protein-bound iodine in plasma was decreased, and the binding of the hormones by plasma proteins was increased. The enhanced rate of metabolism of iodothyronines was associated with stimulation of the binding of these hormones by diverse tissues, suggesting the participation of extrahepatic degradative foci in the increased hormonal deiodination observed in vivo. Increased hepatocellular binding and a significantly enlarged hepatic distribution space of thyroxine were noted. Hepatocellular binding of triiodothyronine was similarly augmented, and a smaller but significant increase in the hepatic space of this iodothyronine was detected. Analysis of the hepatic subcellular partition of iodothyronines 35 min after the intravenous administration of isotopically labeled thyroid hormones disclosed increased hormonal binding by the microsomal fraction in cold-adapted animals and an attendant increase in the microsomal protein concentration. Partial microsomal subfractionation in a discontinuous sucrose gradient indicated that the observed stimulation of microsomal hormonal binding was associated with proliferation of the smooth endoplasmic reticulum.