Rats were exposed to 4 °C and were then made hyperthyroid or hypothyroid by injecting 100 μg l-thyroxine (T4) subcutaneously daily or by giving 0.05% methimazole in lieu of drinking water. Rats, similarly treated, were maintained at 27 °C. Untreated euthyroid animals were also placed at both temperatures. The urinary excretion rates of norepinephrine (NE), epinephrine (E), and their major metabolites were measured for a 10- to 17-day period, after which the animals were killed and tissue catecholamines measured. At 4 °C, the methimazole-treated group failed to adapt as hypothermia developed and several deaths ensued, despite a dramatic increase (18 times) in excretion of norepinephrine. Cardiac catecholamine levels were very low in this hypothyroid group suggesting (1) that synthesis of NE was insufficient to maintain normal tissue levels or ensure adequate heat production, and (2) that catabolism of intraneuronal and secreted catecholamine may have been altered. At 4 °C, the T4-treated group excreted less NE, E, and metabolites than the hypothyroid and euthyroid groups. At 27 °C, T4 treatment resulted in hyperthermia, hypermetabolism, and several deaths. NE excretion in this group was double that of the control and hypothyroid animals, but cardiac levels of NE were low suggesting a discrepancy between synthesis, secretion, and re-uptake.The experiments demonstrate that the reciprocal relationship between thyroid hormones and catecholamines in heat production during cold adaptation is partial only, and that both are essential.