The Present study was undertaken to indicate the major source of NO by liver cells in vitro. Even at early stages of induction or low LPS concentrations, NO was produced at high rates by LPS(Lipopolysaccharide) on the isolated rat kupffer cells. PMA(phorbol 12-myristate 13-acetate) induced NO formation at low rates in the same cells. IFN-${gamma}$ (Interferon-${gamma}$) alone had not induced NO formation but it stimulated the effects of LPS. Calcium ionophore A23187 caused no stimulatory effect. It suggests that LPS has especially strong NO inducer on the kupffer cells and its mechanism is related to those on macrophage in other organs. In other nonparenchymal liver cells, sinusoidal endothelial cells were not stimulated to produce NO either by inducers of aortic endothelium(A23187, ATP and ADP) or by effectors of macrophages(LPS, IFN-${gamma}$. This results suggest that rat liver kupffer cells appear to be the major source of NO by liver cells in vitro. But in vivo, liver endothelial cells may still be capable of producing NO. Furthermore, kupffer cells may produce factors that facilitate NO production by the endothelial cells.