The effects of graded occlusion of the portal vein upon hepatic arterial blood flow were studied in anesthetized dogs to evaluate the so-called "reciprocal relationship" between portal venous flow and hepatic arterial flow in maintaining oxygenation of the liver. An obstruction that increased portal venous pressure to 20 mm Hg was accompanied by a transient increase in hepatic arterial blood flow without changing the other hemodynamic parameters. Release of portal venous occlusion was accompanied by a transient fall in hepatic arterial blood flow in this group of experiments. Increasing portal venous pressure to 30 mm Hg caused a gradual but progressive fall in arterial pressure accompanied by a significant fall in hepatic arterial blood flow. With complete obstruction of the portal vein there is a reduction in arterial pressure to the same level of portal venous pressure and a significant and sustained diminution in hepatic arterial blood flow. These observations conflict with previously described theories of a relationship between diminution in portal venous flow and increases in hepatic arterial blood flow.