Metabolic studies with substrate limb flux determinations require an accurate and simple method for measuring blood flow through the extremity. Two common, noninvasive methods used in recent studies are strain-gauge and capacitance venous occlusion plethysmography. The values obtained for forearm blood flow by these two methods are highly correlated in normal resting and seriously ill patients. The use of a fixed percentage of cardiac output as a measure of extremity flow was not indicative of the measured flow by either capacitance or strain-gauge plethysmography. Capacitance and strain-gauge plethysmography are comparable, noninvasive indicators of change in extremity blood flow in humans.