Sympathetic reflex regulation of subcutaneous blood flow (SBF) in the forearm was studied in eight patients with primary hypothyroidism. Diastolic arterial pressure was greater than or equal to 95 mmHg in five patients. SBF was determined by local clearance of Na99mTcO4. Sympathetic vasoconstriction normally seen after lowering the forearm 40 cm below heart level was absent since SBF only decreased by 4% (+/- 7%, P greater than 0.1) during these conditions. In head-up vertical position we noticed a diminished baroreceptor response as SBF at heart level was reduced by 11% (+/- 7%, P greater than 0.1) compared to supine position. After proximal local anaesthesia SBF increased by 351% (+/- 81%, P less than 0.01) and disclosed a normal vasoconstrictor response as SBF was reduced by 53% (+/- 5%, P less than 0.01) during arm lowering. Five of the treated patients were restudied in the euthyroid state. Mean arterial pressure was reduced in mean by 20 mmHg (+/- 6 mmHg, P less than 0.02) during treatment and a significant vasoconstriction was observed both during arm lowering (SBF = -52% (+/- 6%, P less than 0.02)) and in head-up vertical position (SBF = -45% (+/- 11%, P less than 0.02)). In conclusion sympathetic vasoconstrictor activity in adipose tissue is markedly increased in primary hypothyroidism. Sympathetic tone and arterial pressure are reduced during treatment.