In euthyroid rats, maximal sympathetic nervous system stimulation (e.g. during cold exposure) results in a 3- to 4-fold increase in brown adipose tissue lipogenesis, a response that is blunted in hypothyroid rats. To further investigate this phenomenon, the role of local type II 5′-deiodinase (5′-DII) was studied in freshly isolated brown adipocytes. In a typical experiment, 1.5 × 10⁶ cells were incubated for up to 48 h in a water-saturated 5% CO₂-95% O₂ atmosphere. After incubation with medium alone or with different concentrations of T₄, T₃, and/or norepinephrine (NE), lipogenesis was studied by measuring 1) the rate of fatty acid synthesis as reflected by ³H₂O incorporation into lipids and 2) the activity of key rate-limiting enzymes, i.e. acetyl coenzyme A carboxylase and malic enzyme, and the results are reported in terms of DNA content per tube. Lipogenesis decreased progressively over time (∼40%) when no additions were made to the incubation medium. T₄ or T₃ partially prevented that inhibition at physiological concentrations (65 × 10⁻⁹ and 0.77 × 10⁻⁹m, respectively), whereas a receptor-saturating concentration of T₃ (154 × 10⁻⁹m) doubled the lipogenesis rate. The addition of 10⁻⁶m NE inhibited lipogenesis acutely (∼50% by 12 h) and was followed by a progressive stimulation that reached ∼2-fold by 48 h, but only in the presence of T₄. Furthermore, NE did not attenuate T₃ (154 × 10⁻⁹m)-induced lipogenesis. Both the inhibition and the stimulation of lipogenesis caused by NE showed a strong dose-response relationship within the range of 10⁻¹¹-10⁻⁵m. The role of local 5′-DII was further tested by incubating brown adipocytes with 10⁻⁶m NE and T₄ (65 × 10⁻⁹m) in the presence of 100 μm iopanoic acid, a potent inhibitor of 5′-DII. Although iopanoic acid did not affect the T₃ stimulation of lipogenesis, it did block the ∼2-fold stimulation of lipogenesis triggered by NE in the presence of T₄, confirming the mediation of 5′-DII in this process. In conclusion, lipogenesis in brown adipose tissue is under complex hormonal control, with key roles played by NE, thyroid hormones, and local 5′-DII. As in other tissues, NE-generated signals acutely (12 h) inhibited lipogenesis. However, the presence of the 5′-DII generated enough T₃ to stimulate lipogenesis and gradually reverse the short-lived NE-induced inhibition, leading to the 2- to 3-fold response observed at later time points.