Utilizing struggling to a repetitive stimulation of the tail, as a pain response, flexor reflex to a single stimulation of the sciatic nerve, as a spinal reflex activity, the analgesic activity and the muscle relaxant activity of certain analgesics were assessed alternately. Stable and reproducible response to a repetitive stimulation of the tail or a single stimulation of the sciatic nerve was obtained over a period of 5 hr, and quantitative measurement of struggling could be made. The 50% inhibitory doses of various drugs to struggling and flexor reflex were as follows (mg/kg, i.p.): morphine, 3.30 & 6.60, respectively; codeine, 12.80 & 24.00; pentazocine, 7.80 & 12.20; indomethacin, 4.60 & no effect; aspirin, 94.00 & no effect; baclofen, 1.04 & 2.20; chlorpromazine, 2.45 & 0.75; diazepam, 0.45 & 0.86; mephenesin, 36.00 & 32.00. Based on these results, it is suggested that the method is useful for quantitative measurement of analgesia and muscle relaxation.