Sympathetic skin response and plethysomography were used to assess the sympatholytic effect of the stellate ganglion block with morphine and bupivacaine in patients affected by a sympathetic algodystrophy of the upper limb. Morphine did not show any analgesic or sympatholytic effect. The distribution of the sympatholytic effect and the clinical findings of pain relief without somatic sensory-motor impairments obtained with the local anaesthetic, support the presence of a blockade of the sympathetic efferent pathway as well as of an afferent contingent of fibres probably and mainly involved in sympathetic reflexes.