The effects of long-term cold exposure on muscle and liver mitochondrial oxygen consumption in hypothyroid and normal rats were examined. Thyroid ablation was performed after 8-week acclimation to 4°C. Hypothyroid and normal controls remained in the cold for 8 additional weeks. At the end of 16-week cold exposure, all hypothyroid rats were alive, normothermic and had normal body weight. At ambient temperature (24°C) thyroid ablation induced a 65 % fall in muscle mitochondrial oxygen consumption, which was reversed by thyroxine (T4) but not by norepinephrine (NE) administration. After cold-acclimation was reached, suppression of thyroid function reduced muscle mitochondrial respiration by 30 % but the hypothyroid values remained about 3-fold higher than those in hypothyroid muscle in the warm. Blockade of beta- and alpha-1-adrenergic receptors in both hypothyroid and normal rats produced hypothermia in vivo and a fall in muscle, liver and brown adipose tissue (BAT) mitochondria respiration in vitro. In normal rats, cold-acclimation enhanced muscle respiration by 35 %, in liver 18 % and in BAT 450 % over values in the warm. The results demonstrate that thyroid hormones, in the presence of NE, are major determinants of thermogenic activity in muscle and liver of cold-acclimated rats. After thyroid ablation, cold-induced nonshivering thermogenesis replaced T3-induced thermogenesis and normal body temperature was maintained.