Epididymal adipose tissue in the rat is generally considered to be "pure" white adipose tissue (WAT) with a characteristic structure and function. Previous studies in cats have, however, indicated that adipose tissue with the morphological appearance of WAT could be converted into a tissue with the morphological appearance of brown adipose tissue (BAT) by intermittent cold stress. The present electron microscopic and morphometric study describes the effect of intermittent cold stress on the epididymal WAT of young rats. The tissue volume decreased markedly as did the lipid content. The mitochondrial volume increased dramatically. The extracellular matrix was vastly reduced as was the thickness of the plasma membrane, and the number of gap junctions between adipocytes increased markedly. Indications of neoinnervation and neovascularization were observed. A great abundance of preadipocytes indicated proliferative activity of the endothelium. The low amount of lipid droplets and a relative abundance of smooth and rough endoplasmic reticulum. Golgi apparatus, and lysosomes in the epididymal WAT of cold-stressed rats gave the cells the morphological appearance of young adipocytes or preadipocytes whereas the hypertrophic and hyperplastic mitochondria, the relative paucity of ribosomes on lipid droplet membranes, and the increased innervation and vascularization gave the cells the morphological characteristics of brown adipose tissue.