The blood flow in the sciatic nerve of the rabbit was estimated from the wash out of intraneurally injected133Xe. To avoid diffusion of the tracer into the surrounding muscular tissue, the nerve was covered by a gas-tight plastic film. Using this technique, the basal blood flow in the sciatic nerve was estimated to 35 ml · min−1· 100 g−1. It was found that intraarterial norepinephrine and electrical stimulation of the lumbar sympathetic chain strongly reduced the wash out of133Xe, which only can be explained by a pronounced reduction of the blood flow in the nerve itself. The blood flow again increased within 4 min of stopping the infusion of norepinephrine or the sympathetic stimulation. The prolonged effect and higher neurotoxicity of local anesthetics containing adrenaline may be explained by an alpha receptor-mediated vasoconstriction of the microvessels of peripheral nerves.