Changes in skin blood flow in a digit following sympathetic denervation were determined by use of a photoelectric digital plethysmograph (pulse-wave monitor). Results in 20 cases of diagnostic sympathetic block, surgical sympathectomy, or both, are discussed. Pulse-wave changes were seen 3 to 5 minutes after block and 1/2 to 1 minute after sympathectomy. Use of a pulse-wave monitor permitted limiting the dosage of local anesthetic, usually to 5 to 10 ml. of 0.5 percent lidocaine for lumbar sympathetic block and 1 to 2 ml. of 0.5 percent lidocaine for stellate ganglion block. Pulse-wave monitoring was found to be simple and rapid.