Arterial and venous blood flow and oxygen saturation of the venous blood in the mesenteric vessels arising from single mesenteric artery and vein to the intestinal loop in anesthetized dogs were measured simultaneously with systemic arterial blood pressure during distension of this loop. Mesenteric arterial blood flow decreased to the zero level, while mesenteric venous outflow increased to about twice the quiescent level at the onset of elevation of intraluminal pressure. However, during distension, the decreased arterial blood flow tended to increase, but the increased venous flow was reduced to one half of the quiescent level. Oxygen saturation of venous blood diminished initially and then increased progressively during distension. At the end of distension venous outflow stopped temporarily. After suspension of distension, arterial and venous flow and oxygen saturation increased beyond the quiescent level. These results may suggest that there is a by-pass or A-V anastomosis which cannot be compressed mechanically by elevation of intraluminal pressure.