1. Plethysmographic blood flow records made after venous occlusion of the forearm showed a biphasic response which was first vasodilator and then vasoconstrictor.
2. The myogenic nature of the vascoconstrictor phase was confirmed in eight subjects after total autonomic blockade with atropine, propranolol, phentolamine and guanethidine.
3. Forearm venous blood demonstrated a rise in hydrogen ion concentration and a fall in oxygen tension during venous occlusion, which may contribute to the vasodilatation phase.