Hepatic dysfunction after portacaval shunting (PCS) has been attributed to loss of portal perfusion to the liver. Proponents of selective systemic shunting state that reduced encephalopathy and hepatic dysfunction with this procedure result from the maintenance of portal perfusion to the liver through the hypertensive mesenteric venous circulation. We questioned the importance of maintaining the diminished portal flow to the cirrhotic liver because hepatofugal flow is known to develop in many of these patients. We sought to further define mechanisms that may contribute to the maintenance of critical flow to the liver in compensated hepatic cirrhosis. We demonstrated a primary relationship between mesenteric venous hypertension (MVH) and increased hepatic arterial blood flow after diversion of portal blood flow. Fifteen dogs had vena caval stenosis above an end-to-side PCS to establish MVH and deprive the liver of portal blood flow. Another 15 dogs had end-to-side PCS alone. A half hour after shunting, hepatic arterial blood flow had increased significantly in all dogs. Hemodynamic parameters remained stable throughout. Six weeks later, mesenteric pressure increased 98% +/- 3% with intracaval stenosis (from 9.6 +/- 0.1 to 19.0 +/- 0.3 cm H2O). Mesenteric pressure was unchanged with PCS alone (9.0 +/- 0.1 cm H2O). Increased hepatic arterial flow was significantly elevated in all dogs above pre-shunt values by 6 weeks postshunt. With MVH, however, further augmentation in hepatic arterial flow was noted in the chronic state (1.5 +/- 0.1 vs 0.9 +/- 0.1 ml/min/gm, p less than 0.05). There was significant correlation between MVH and increased hepatic arterial flow in the chronic state (r = 0.79, p = 0.05). Hepatic arterial flow 6 weeks after PCS with MVH was associated with lower blood ammonia and improved hepatocellular function compared with animals with PCS alone. These results support the hypothesis that MVH is important in maintaining blood supply--beyond providing driving force for sustained portal flow to the liver. This is an important consideration in the medical and surgical management of portal hypertension, a condition in which profound reduction in portal pressure may negatively affect compensatory hepatic arterial blood flow.