Fluid shear stress can be increased either by increasing the flow rate or perfusing increasing doses of norepinephrine (NE) at a constant flow rate. Concomitantly, increased fluid shear stress at the surface of endothelial cells releases nitric oxide (NO). To better understand the role of NO released by shear stress in regulating intrahepatic vascular resistances, we increased fluid shear stress either by changing the flow rate or by perfusing increasing doses of NE at a constant flow rate in perfused livers isolated from normal rats. When concentration-response curves to NE were studied at low, mild, and high flow rates, portal pressure increased during NE perfusion. The higher the flow rate, the lower the response to NE. NO synthase inhibition similarly increased the response to NE at each flow rate. Thus, NO was released by NE-induced increased shear stress, but other vasodilators are likely to be responsible for the flow-induced increased shear stress. In additional experiments, when flow rate was decreased while infusing increasing doses of NE to maintain the portal pressure constant, shear stress remained steady and NO was not released. Hepatic NO production in the different conditions of shear stress could not be detected. Our data are consistent with the fact that in the liver, NO released by shear stress decreases the vasoconstriction to NE and regulates the intrahepatic vascular resistances.