Interscapular brown adipose tissue was removed from young rats, the cells isolated using collagenase, pelleted by centrifugation and cultured. Ultrastructural observations were made on these cells immediately after isolation and on Days 1, 2, 4, 10 and 14 of culture. The isolated cells show the typical submicroscopic features of cells in intact brown adipose tissue, i.e. a multivacuolar lipid depot and numerous typical mitochondria with dense cristae. However, as early as Day 1 of culture, the cells show degeneration of the typical mitochondria which are probably then extruded from the cells. Atypical mitochondria now begin to appear and become dominant on Day 4 of culture. By contrast, the typical multivacuolar shape of the lipid droplet does not change. Not even after prolonged periods of culture (2-4 weeks) is there any evidence of coalescence of the multivacuolar lipid depot. These findings are consistent with the possibility that the typical mitochondria of brown adipocytes depend on in vivo conditions, perhaps adrenergic innervation, while the multivacuolar characteristic of the lipid depot does not, and may be an intrinsic feature of the brown adipose cell.