The adrenergic nerve fibers of brown adipose tissue of warm- and cold-acclimated rats were visualized using the fluorescence histochemical technique. Small arteries and arterioles evidenced by their rich adrenergic innervation appeared more abundant in tissue from cold-acclimated animals. Lengths of fine fibers between the fat cells were demonstrable in the brown fat tissue from cold-acclimated rats because of their abundant, strongly fluorescent varicosities whereas such portions of fine fibers were only occasionally seen in the tissue of warm-acclimated rats.