Mechanical contraction of the dorsal sarco-cocygeus muscle and electrical stimulation of spinal cord elicited tail elevation such as straub tail reaction (STR). When morphine, 0.25 approximately 0.5 microgram, was injected into the lumbothecal space, STR was produced dose-dependently and such was the same as that produced by systemic injection of morphine. STR was not observed, when morphine was injected into the lumbo-thecal space in spinal mice. STR was antagonized by tubocurarine given subcutaneously and by naloxone injected into the lumbothecal space. STR was not antagonized by the right, left or both dorsal spinal lesion at C5 approximately 6 as well as by the right or both dorsal spinal lesion at T11 approximately 12. However, STR was antagonized by the spinal transection at T11 approximately 12. The lesion of spinal serotonergic neurons enhanced STR. These results suggest that morphine acts on both the lumbo-sacral nerve cell bodies and the nerve terminals descending via the spinal ventral horn, by which the morphine-induced contraction of dorsal sarco-coccygeus muscle tendons produce STR.