The role of the sympathetic nervous system in the regulation by insulin of the level of uncoupling protein in brown adipose tissue has been examined. The amount of uncoupling protein was substantially reduced in streptozotocin‐diabetic rats, while insulin replacement to diabetic animals induced a partial restoration. Unilateral denervation of the interscapular brown fat pads also lowered the amount of uncoupling protein, and in diabetic animals inhibited the stimulation of the level of the protein by insulin replacement. Maintenance of normal uncoupling protein levels requires both insulin and the sympathetic system; regulation of the protein by insulin involves sympathetic mediation.