The pressure-flow relationship was studied in isolated denervated intestinal loops of the dog in a range of pressure varying from 0 to 240 mm. Hg. The relationship was curvilinear, convex to the pressure axis. Flow started at a minimal pressure of 16 mm. Hg, then increased more rapidly than pressure up to 240 mm. Hg. Critical closure, passive expansion of vessels with rising intraluminal pressure, and anomalous viscosity of blood appeared to be the important factors influencing vascular resistance and flow in the present studies.