We have used a telemetry system to record heart rate, body temperature, electrocardiogram (ECG), and locomotor activity in awake, freely moving mice lacking thyroid hormone receptor (TR)-β or TR-α₁ and -β (TR-α₁/β). The TR-α₁/β-deficient mice had a reduced heart rate compared with wild-type controls. The TR-β-deficient mice showed an elevated heart rate, which, however, was unresponsive to thyroid hormone treatment regardless of hormonal serum levels. ECG revealed that the TR-β-deficient mice had a shortened Q-T_end time in contrast to the TR-α₁/β-deficient mice, which exhibited prolonged P-Q and Q-T_end times. Mental or pharmacological stimulation of the sympathetic nervous system resulted in a parallel increase in heart rate in all animals. A single injection of a nonselective β-adrenergic-receptor blocker resulted in a parallel decrease in all mice. The TR-α₁/β-deficient mice also had a 0.4°C lower body temperature than controls, whereas no difference was observed in locomotor activity between the different strains of mice. Our present and previous results support the hypothesis that TR-α₁ has a major role in determining heart rate under baseline conditions and body temperature and that TR-β mediates a hormone-induced increase in heart rate.