The influence of body posture on limb blood flow was studied in normal subjects, in patients with arterial insufficiency of the lower limbs and in hypertensive patients using the <sup>133</sup>Xe local clearance technique. Muscle blood flow in the three groups, measured at rest or after ischaemia provoked by cuff, with or without exercise, was significantly lower in the erect position (sitting or standing) compared to the recumbent position. Both in normal subjects and in patients with arterial insufficiency muscle blood flow, computed from the disappearance rate of <sup>133</sup>Xe, injected together with histamine into the tibialis anterior muscle, was not significantly different in the recumbent and sitting position. On the contrary, in both groups of subjects the skin blood flow computed from the disappearance rate of <sup>133</sup>Xe, injected together with histamine intradermally on the dorsum of the feet, was significantly higher in the sitting position than in the recumbent position. These results are discussed in connection with the relief of pain observed by certain patients with arterial insufficiency when they dangle their legs.