In this study, we examined whether matrine could inhibit the differentiation of 3T3-L1 preadipocytes and further explored the possible inhibitory mechanisms. Evidenced by Oil Red O staining and AdipoRed assay, matrine dose-dependently inhibited lipid accumulation at concentrations of 125, 250 and 500 mu g/ml. At molecular level, the expression of transcription factors, PPAR gamma and C/EBP alpha, was reduced by matrine during adipogenesis. After treatment for 6 days, the mRNA levels of adipocyte-specific genes, such as aP2, LPL, adiponectin and leptin, were also down-regulated by matrine in a dose-dependent manner. Moreover, 500 mu g/ml matrine inhibited the phosphorylation of ERK1/2 at the early stage of differentiation. Our results indicate that inhibition of 3T3-L1 preadipocyte differentiation by matrine is associated with the suppression of ERK1/2 phosphorylation. Thus, matrine has the potential to be an alternative natural product for the treatment of obesity. (C) 2010 Elsevier Inc. All rights reserved.