Guggulsterone (GS) and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] have been shown to influence adipogenesis in 3T3-L1 cells. We investigated the ability of GS and 1,25(OH)2D3, alone and in combination to inhibit adipogenesis and induce apoptosis in 3T3-L1 adipocytes. Maturing preadipocytes were treated with 1,25(OH)2D3 in combination with GS for 6 days during differentiation. GS and 1,25(OH)2D3 each inhibited lipid accumulation, but the combination potentiated the inhibition of lipid accumulation. Apoptosis was increased by 1,25(OH)2D3 while GS had no effect, but GS + 1,25(OH)2D3 increased apoptosis more than either compound alone. Furthermore, GS + 1,25(OH)2D3 caused a potentiated decrease in the expression of aP2 and farnesoid X receptor expression more than either compound alone. In addition, 1,25(OH)2D3 increased vitamin D receptor expression after 6 days, while GS had no effect. GS + 1,25(OH)2D3, however, caused a potentiated increase in the expression of VDR. These findings show that GS potentiates 1,25(OH)2D3's anti-adipogenic and pro-apoptotic effects in maturing 3T3-L1 preadipocytes. © 2007 Elsevier Inc. All rights reserved.