ABSTRACT— To clarify the relation of pericentral fibrosis to portal hypertension, measurements of portal vascular resistance in vitro and blood pressures of several key points in hepatic vascular pathways in vivo were undertaken in rats given dimethylnitrosamine. Administration of dimethylnitrosamine induced tortuosity and narrowing of the peripheral branches of the hepatic vein due to pericentral fibrosis. No significant change was produced in the sinusoids and the portal vein branches. The portal vascular resistance was increased and the portal vein pressure was elevated markedly. The blood pressure gradient was steep in the intrahepatic vein, but not in the intrahepatic portal vein or the sinusoids, as compared to control. These data suggest that deformation of the peripheral branches of the hepatic vein due to pericentral fibrosis causes a marked increase in vascular resistance in the intrahepatic hepatic vein, i.e. postsinusoidal portal hypertension.