1. Lumbar sympathectomy creates a state of vasodilatation equivalent to physiological vasodilatation of daily living; however, after sympathectomy, vasodilatation is permanent and irreversible. 2. This vasodilatation is characterized at rest by a preferred distribution of blood to the skin and to subcutaneous cellular tissue of the lower extremity, foot and toes. 3. This vasodilatation, although permanent, does not interfere with increased output to muscles at work. They benefit by a particular mechanism of metabolic vasodilatation (Fig. 6). 4. Lumbar sympathectomy has a beneficial effect on the collateral compensatory circulation of an obliterated large artery. This effect is induced by suppression of the peripheral resistance, and thus, by increasing the pressure gradient. 5. The vasodilatation obtained by intravenous injection of pentothal is qualitatively the same as that which follows a lumbar sympathectomy. The test with pentothal, because of this action, is an excellent method for prognosis and control of lumbar sympathectomy effects.