1. Plethysmographs containing the hand plus forearm were used to measure blood flow in patients with a srugically created arteriovenous fistula in one forearm. 2. Apparent flow rate was stable over a limited range of collecting pressures; the absolute value of these pressures varied from patient to patient. 3. After arterial occlusion, blood flow increased by a similar amount on the normal side and on the side with the fistula. 4. Occlusion of fistular flow produced no significant change in heart rate. 5. Fistular flow, estimated as the difference between flow on the two sides, averaged 525 ml/min in seventeen patients.