Variations in flow in the presence of a constant arteriovenous pressure difference can only occur when vascular dimensions have been altered, irrespective of the rheological properties of the liquid being perfused. When the vascular bed of the hind leg is subjected to parallel changes in arterial and venous pressures, higher absolute values of pressure result in an enhancement of flow. This constitutes convincing evidence to indicate that certain vessels which offer a significant resistance to blood flow are distensible.