1. In eight patients with a unilateral fistula between the radial artery and a nearby superficial vein, heat elimination from both hand and forearm, as measured by calorimetry, was always substantially greater on the side of the fistula (mean excess from hand-plus-forearm 889 J/min).
2. Fistular blood flow measured by hand-plus-forearm plethysmography in these patients averaged 431 ml/min. Correlation between fistular blood flow and heat elimination was poor (r = 0.70, P < 0.06), probably because heat elimination due to the fistula takes place mainly from veins, whose pattern varies from patient to patient.
3. Approximately half of the total increased heat elimination due to the fistula is from the hand. Occlusion of the circulation to the hand caused fistular flow rate to be reduced by about half. This suggests that the main resistance to fistular flow is venous, proximal veins offering a similar resistance to distal veins.
4. The obligatory heat loss due to the fistula is unlikely to embarrass temperature regulation, except in severe cold stress.