Background /Aims: Reactive oxygen species (ROS) induce HSCs activation, proliferation and collagen gene expression in vitro. Nitric oxide (NO) represents a reactive molecule that reacts with ROS, yielding peroxynitrite. We thus verified the effect of NO on ROS‐induced HSCs proliferation in vitro and correlated iNOS expression and ROS formation to HSCs activation in the early phase of liver injury leading to hepatic fibrosis in vivo. Methods/Results: HSCs were incubated with iron ascorbate (FeAsc) in vitro, which induced ROS production, ERK1/2 phosphorylation and increased cell proliferation. This effect was significantly reduced by the presence of the NO donor S‐nitroso‐N‐acetylpenicillamine. Liver injury was induced in vivo in rats by dimethylnitrosamine administration. HSCs activation started 6 h after DMN administration and peaked at 1 week. ROS generation and neutrophil infiltration were evident for at least 48 h after DMN treatment, showing an identical distribution pattern. Only a few inflammatory cells expressed iNOS 6 h after DMN administration. Conclusions: we have shown that NO acts as a ROS scavenger in vitro, thus inhibiting HSCs proliferation. ROS production by infiltrating neutrophils occurs in the early phase of liver fibrosis and can represent a stimulus to HSCs activation in vivo. The reduced iNOS expression may account for the low NO levels and the inability to prevent the ROS‐induced HSC activation in vivo.