The administration of morphine was followed in white mice by a typical Straub reaction which consisted of the tail becoming rigid and erected across the back of the animal in an S-shaped curve. This reaction was accompanied by restlessness, excitability, extension rigidity of the hindlimbs, forcible viodance of faeces and prominence of the perineum. The Straub reaction was abolished by general anaesthesia with pentobarbitone or ether, by administration of tubocurarine, by bilateral section of the muscles causing extension to the tail, and by the removal of the circulation to the lower extremity. The reaction was modified by unilateral section of the extensor muscles of the tail. Section of the spinal cord, decortication, division of the anal sphincter and perineal floor, or ablation of the pelvic splanchnic nerves did not suppress the appearance of the Straub response. It was concluded that the phenomenon described by Straub (1911) was produced mainly by the action of the sacro-coccygeus dorsalis muscle, and that it was also necessary that the lumbo-sacral cord with its peripheral nervous outflow should be intact and that these functioning units should have an adequate circulation.