White and brown adipocytes are usually located in distinct depots; however, in response to cold, brown adipocytes appear in white fat. This response is mediated by beta-adrenoceptors but there is a controversy about the subtype(s) involved. In the present study, we exposed to cold beta 3-adrenoceptor knockout mice (beta 3KO) on a C57BL/6J genetic background and measured in white adipose tissue the density of multilocular cells and the expression of the brown adipocyte marker uncoupling protein-1 (UCP1). In brown fat of beta 3KO mice, UCP1 expression levels were normal at 24 degrees C as well as after a 10-day cold exposure. Strikingly, under both conditions, in the white fat of beta 3KO mice the levels of UCP1 mRNA and protein as well as the density of multilocular cells were decreased. These results indicate that beta 3-adrenoceptors play a major role in the appearance of brown adipocytes in white fat and suggest that the brown adipocytes present in white fat differ from those in brown fat.