Modification of histone H3 at lysine 9 from methylations to acetylation is important for transactivation of genes. In this study, we found that all methylations (mono-, di-, tri-) of histone H3 at lysine 9 on the adiponectin gene decreased by stimulating adipocyte differentiation prior to increases in adiponectin gene expression and acetylation of histone H3 at the same residue on the gene during adipocyte differentiation of 3T3-L1 cells. Additionally, we revealed that decrease of adiponectin gene expression by treatment with TNFalpha, an inducer of insulin resistance in adipocytes, was associated with decreased acetylation of histone H3 at lysine 9 on the gene, but not methylations. Our results suggest that induction of the adiponectin gene during adipocyte differentiation is associated with modification of histone H3 at lysine 9 from methylations to acetylation, whereas reduction of the adiponectin gene in 3T3-L1 adipocytes with insulin resistance is associated with decreased acetylation at lysine 9 of histone H3.