BACKGROUND: Rodent brown adipose tissue (BAT) is considered the main effector of adaptative thermogenesis as it contains a unique mitochondrial uncoupling protein, termed as uncoupling protein-1 (UCP1). The emergence of ectopic brown adipocytes in the white adipose tissue (WAT), called recruitment, might play an important role in the prevention of obesity. The recruitment phenomenon has until now been investigated mostly in vivo. OBJECTIVES: This study is an attempt to mimic in vitro the recruitment phenomenon. It consisted in culturing the stroma vascular fractions of mouse BAT and WAT in a brown adipocyte differentiation medium. The multilocular cells obtained, referred to as BAT(B) and WAT(B) adipocytes, respectively, were compared. RESULTS: The BAT(B) and WAT(B) adipocytes were morphologically different. The expressions of UCP1, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), leptin and resistin mRNAs were low in WAT(B) adipocytes as compared with those in BAT(B) adipocytes. The expressions of UCP1 and PGC-1alpha proteins were, however, much higher in WAT(B) adipocytes, amounting 51% and 36% of those in BAT(B) adipocytes. The patterns of expression of UCP1, PGC-1alpha and leptin in the BAT(B) and in WAT(B) adipocytes were different with a higher relative expression of PGC-1alpha in the latter. Rosiglitazone increased UCP1 mRNA expression 4.5-fold in the BAT(B) and significantly more, 7.9-fold, in the WAT(B) adipocytes. Retinoic acid and triiodothyronine increased UCP1 mRNA expression in the BAT(B) adipocytes 1.6- and 2-fold, respectively but, surprisingly, slightly decreased UCP1 mRNA expression in the WAT(B) adipocytes. CONCLUSIONS: The study suggests that the nature and possibly the origin of WAT brown adipocytes is different from that of BAT brown adipocytes. It proposes an in vitro approach that could prove very useful to better characterize the WAT brown adipocyte-like cells.