In a group of patients suffering from reflex sympathetic dystrophies, the skin potential and EMG responses induced by electrical stimuli applied to the skin were recorded in the four limbs in order to study somato-sympathetic and somato-motor reflexes. In most patients, the amplitude, delay and shape of the cutaneous responses as well as the pattern of the EMG responses were different from those observed in normal subjects. In particular, it was possible to correlate the pattern of the cutaneous and muscular responses with the severity of the disease. The cutaneous sensory thresholds to electrical stimuli (tactile, tingling and pain threshold) showed different values in the dystrophic and in the contralateral limb. In all patients, a block of the sympathetic chain ipsilateral to the dystrophic limb was performed with local anesthetics. 1 h after the block, the cutaneous responses disappeared not only in the blocked limb but also in the contralateral limb. 48 h after the block, muscular and cutaneous responses as well as sensory thresholds showed a pattern similar to that observed in normal subjects. These findings show that the sympathetic block provides a resetting of the sensory thresholds and reflexes.