Mitochondrial uncoupling protein 1 (UCP1) is a speci®c marker of multilocular brown adipocytes. Ectopic UCP1 in white fat of aP2-Ucp1micemitigates development of obesity byboth, increasing energy expenditure anddecreasingin situ lipogenesis. In order to further analyse consequences of respiratory uncoupling inwhite fat, the e ects of the ectopic UCP1 on the morphology of adipocytes and biogenesis of mitochondria in these cells were studied.