Measurement of blood flow through the portal vein of the dog revealed a reduction of flow and an increase in vascular resistance of the mesenteric bed in proportion to the elevation of portal venous pressure. To obtain further information on this mechanism, studies were performed on segments of terminal ileum with continuous measurement of both weight and flow. These studies demonstrated that the increase of vascular resistance was not dependent upon extrinsic or adrenergic innervation, and did not appear to be secondary to interstitial fluid formation. It is suggested that the response in this bed is due to a local reflex to a myogenic reaction of the resistance vessels to increased transmural pressure.