A light and electron microscopic study of a hibernoma and a brief discussion of the functional role of brown fat are presented. The multivacuolar cells composing brown fat have central round nuclei with prominent nucleoli. The quantity of mitochondria and lipid droplets in the cytoplasm of the cells varies considerably. The electron microscopic findings of hibernoma include multivacuolated and univacuolated cells with varying numbers of lipid vacuoles, lysosomes, lipofuscin granules, pinocytotic vesicles, and possibly prominent subplasmalemmal condensations. Interesting findings in this case include the presence of amorphous diastase resistant perodic acid (PAS) Schiff positive peripheral cytoplasmic masses which are variable in size and shape. These collections also appear extracellularly. The presence of these masses, presumably composed of neutral mucopolysaccharides, has not, to our knowledge, been recorded previously.