The present studies were carried out because of a need for a better understanding of the functional capacity of the collateral circulation in health and in disease. Aortography has been an important method of obtaining anatomic evidence of the collateral circulation 1 ; however, little attention has been paid to its functional effects. As the presence of collateral circulation is at times highly important and may be the only circulation sustaining a limb, it would be desirable, if possible, to measure the degree of collateral circulation to a limb. The present studies were carried out to determine (1) if measurements of the circulation through major and collateral channels could be made and (2) differences in the amount of collateral circulation in normal subjects and in patients with arteriosclerosis obliterans. Methods and Materials Two plethysmographic methods were employed: I. The first compared the volume (height) of the pulsation of a digit with