This study aimed to investigate the effects of reactive oxygen species on the hepatic macrophages, the Kupffer cells (KC), and to identify the relevant targets of vasoconstrictors involved in the regulation of intrahepatic microcirculation and therefore portal pressure. The effects of hydrogen peroxide (H2O2), xanthine/xanthine oxidase or a thromboxane (TX) analogue (U46619; 0.1 microM) were tested in sham-operated and fibrotic livers (bile duct ligation over 4 weeks) during isolated rat liver perfusion and in vivo with or without additional KC blockade (gadolinium chloride, 10 mg kg(-1) body weight, 48 and 24 h, i.p.). To investigate downstream mechanisms, a TXA2 antagonist (BM 13.177; 20 microM) or a Rho kinase inhibitor (Y27632; 10 microM) was infused additionally. TXB2 efflux was measured by enzyme-linked immunosorbent assay. The phosphorylation state of moesin (p-moesin), as indicator for Rho kinase activity, was assessed by Western blot analyses. Portal pressure was dose-dependently increased by H2O2 (maximum, 0.5 mM) and, to a lower extent, by xanthine/xanthine oxidase together with catalase. The portal pressure increase by H2O2 was attenuated by previous KC blockade. TXA2 efflux increased after H2O2 infusion and was reduced by KC blockade. The TXA2 antagonist counteracted the H2O2-induced increase in portal pressure. The Rho kinase inhibitor attenuated portal pressure increase after TXA2 analogue or H2O2 infusion. Hepatic levels of p-moesin were increased after H2O2 infusion. Reactive oxygen species increased portal pressure via stimulation of TXA2 production by KCs and a subsequent Rho kinase-dependent contraction of the intrahepatic vasculature. In conclusion, the KCs that are well known to produce H2O2 could also be activated by H2O2. This vicious cycle may best be interrupted at the earliest time point.