The effect of butyrate, a natural bacterial product of colonic bacterial flora, on nitric oxide (NO) production in murine vascular endothelial cell line END-D in response to IFN-gamma and/or LPS was studied. Butyrate significantly augmented NO production in END-D cells in response to IFN-gamma or IFN-gamma + LPS, but not LPS alone. The NO production was augmented by the addition of butyrate until 6 h after the stimulation with IFN-gamma or IFN-gamma + LPS. The augmentation was abolished by the removal of butyrate from the cultures. Butyrate enhanced the expression of inducible type NO synthase (iNOS) in the stimulated END-D cells. Furthermore, butyrate-enhanced NO production in the presence of various signal inhibitors down-regulating the signal pathways using nuclear factor (NF)-kappaB, mitogen-activated protein (MAP) kinases and Janus tyrosine kinase. The putative mechanism of butyrate-induced augmentation of NO production in response to IFN-gamma or IFN-gamma + LPS is discussed.