1. Venous occlusion plethysmography was used to measure blood flow through the extremities of five normal subjects before and during exposure to a standard heat load with one extremity maintained at a cold (15°C) and the other at a neutral (35°C) temperature.
2. In the foot, local cold (15°C) delayed the onset and reduced by about 92% the vasodilatation during release of sympathetic vasoconstrictor tone caused by body heating.
3. Similar results were obtained in the hand exposed to cold (15°C) but the suppression of the reflex vasodilatation in response to body heating was about 66%.
4. This difference between the hand and foot did not appear to be related to the greater length of the foot since the distal half of the foot reacted like the total foot: local cooling suppressed the reflex vasodilatation by 88%.
5. The results suggest that sympathetic release tests should not be carried out in a laboratory with an environmental temperature as low as 15°C, especially when the circulation to the foot is under investigation.