β-hydroxybutyric acid (BHBA) downregulates NF-κB activation via GPR109A. Primary rat microglial cells were treated with 0 or 10 ng/ml LPS for the indicated times in the presence or absence of 1.5 mM BHBA. Western blot was performed with the indicated antibodies. At 0.5, 1, 2, 4 h after lipopolysaccharide (LPS) stimulation, significant reductions in pNF-κB levels were observed in the BHBA-treated primary rat microglial cells (GPR109A WT) (A, B). In contrast, no difference was observed in pNF-κB levels between the vehicle- and BHBA-treated primary rat microglial cells with the silencing of GPR109A (GPR109A KO) (C, D). Each immunoreactive band was digitized and expressed as a ratio of the β-actin level. The ratio of the control group band was set to 1.00. The data are expressed as the mean ± SD of three independent experiments. ##P <0.01 and #P <0.05 indicated significant differences compared with the no-treatment group (NT). **P <0.01 indicated a significant difference compared with the BHBA-untreated LPS-stimulated group.