We have examined the uncoupling (UCP) protein gene expression in euthyroid and hypothyroid rats. UCP mRNA levels were estimated by northern blot analysis of total RNA from brown adipose tissue (BAT). Stimuli were endogenous (cold) and exogenous norepinephrine (NE), isoproterenol, T₃, and T₄. While in euthyroid rats UCP mRNA levels increase 2- to 3-fold by 2 h after NE or overnight cold exposure, these stimuli and isoproterenol are ineffective in hypothyroid rats. One single dose of T₄, equal to the daily production rate, brings about a normal response in hypothyroid rats exposed to cold overnight. Hypothyroid rats recover their responsiveness to NE approximately 4 h after a receptor saturating dose of T₃. On the other hand, such a dose of T₃ induces a 3- to 4-fold increase in UCP mRNA levels in hypothyroid rats without the need of exogenous NE, and this response is not reduced by raising ambient temperature to thermoneutrality (28 C). However, the following evidence indicates that T₃ requires adrenergic input to stimulate the accumulation of UCP mRNA: first, euthyroid animals maintained at 28 C do not respond to such a treatment. Second, when T₃ was injected to hypothyroid rats with unilaterally denervated BAT, only the intact side responded to T₃ with an elevation of the UCP mRNA levels, but both sides remained responsive to T₃ + NE. We conclude that: 1) the failure of UCP mRNA to increase in response to cold is due to an intrinsic defect in BAT to respond to NE; 2) T₃ cannot induce gene in the absence of adrenergic input; and 3) the full expression of UCP gene requires both adrenergic- and T₃-dependent signals. In addition, the data are consistent with reports indicating an elevated sympathetic tone on BAT in hypothyroidism and, further, suggest that thermal stress is not the only cause for the enhanced sympathetic activity in this condition.