The effects of ginsenoside-Rh1 and Rh2 in the induction of nitric oxide (NO) synthesis by IFN-gamma plus LPS were investigated using murine peritoneal macrophages. The NO production from rIFN gamma plus LPS-treated macrophages was markedly reduced by ginsenoside-Rh1 or Rh2 in a dose dependent manner, but had no inhibitory effects by ginsenoside-Rb1, Rc or Re. In addition, treatment of the cells with ginsenoside-Rh2 6 hr before the stimulation with IFN-gamma plus LPS showed more inhibitory effect than the treatment with ginsenoside-Rh2 6 hr after or simultaneously with the stimulation with IFN-gamma plus LPS in the NO production. Ginsenoside-Rh2 also effectively inhibited IFN-gamma induced NO production when the cells were treated with IFN-gamma 6 hr after the treatment with ginsenoside-Rh2. Our findings suggest that this phenomenon might be caused by inhibition of priming signal such as IFN-gamma for the synergistic induction of NO synthesis.