We studied the role of the sympathetic innervation in development and maintenance of increased levels of uncoupling protein (UCP) and of thyroxine 5'-deiodinase (TD) during cold-induced growth of brown adipose tissue (BAT). Interscapular BAT was unilaterally (and in some experiments, bilaterally) denervated either before acclimation to cold (4 degrees C) for 12 days or after 14 days of a total 28-day period of acclimation to cold. BAT norepinephrine was reduced to 3-7% of the normal level in denervated BAT for up to 26 days. Denervation slowed, but did not prevent, cold-induced increases in total protein, in mitochondrial GDP binding, and in mitochondrial UCP concentration, which all reached 50% or more of the elevated level in intact tissue. In contrast, TD activity did not exceed 10% of the elevated level in intact tissue at any time. Denervation after cold acclimation resulted in a very rapid loss of TD activity, a slower and selective loss (after a lag of 1 day) of UCP, and a much slower loss of tissue protein. We conclude that the sympathetic innervation is required for an optimal trophic response of BAT to cold acclimation and for maintenance in the hypertrophied state but that other factors are also involved. Induction and maintenance of TD in BAT does need the sympathetic innervation.