Wistar rats acclimated to cold received iv 25 micrograms T 4/100 g bw, 21 micrograms T 3/100 g bw or 2.5 micrograms T 3/100 g bw, to observe the changes induced in the peripheral metabolism of 125I-T4 and 125I-T3 relative to cold-adapted untreated animals. Each animal was injected with a tracer dose of either labelled hormone in addition to the T4 or T3 load, and placed in metabolic cages for 24 hour collection of urine and feces. Sequential heparinized blood samples were obtained by cardiac puncture. A T4 load decreased the deiodination of 125I-T4 and 125I-T3 (p less than 0.01) as revelated by urinary 125I. A T3 load, in the two dosages employed, decreased the deiodination of 125I-T3 (p less than 0.01) but had no effect on deiodination of 125I-T4. Similarly, a T4 load increased the fecal excretion of both radioactive iodothyronines (p less than 0.01) whereas a T3 load failed to alter the excretion of 125I-T4. In cold-adapted animals plasma TSH was elevated (p less than 0.05) and plasma T4 was low (p less than 0.001) as compared to rats housed at 22 C. It is concluded that the relative contribution of T4 and T3 to the metabolic state in the rat is not significantly altered by cold exposure.