Evidences are presented to show a strong and long-lasting analgesic effect after injec-tion of dynorphin into the subarachnoid space of the spinal cord in the rat. Taking theamplitude and time course of the increase of tail flick latency as the indices of analgesia,dynorphin elicited dose-dependent analgesic effect in the range of 2.3--18.6 nmol. Calcu-lating on a molar basis dynorphin was 6--10 times more potent than morphine and 65--100 times more potent than morphiceptin, another mu opiate receptor agonist. Dynorphinanalgesia was completely reversed by intrathecal injection of anti-dynorphin IgG and par-tially reversed by naloxone. Acute tolerance to morphine analgesia did not affect the occu-rance of dynorphin analgesia, indicating the absence of cross tolerance between morphineand dynorphin. Evidence from different lines of approach suggests that dynorphin may bindwith kappa opiate receptors in the spinal cord to exert its analgesic effect Evidences are presented to show a strong and long-lasting analgesic effect after injection of dynorphin into the subarachnoid space of the spinal cord in the rat. Taking the amplitude and time course of the increase of tail flick latency as the indices of analgesia, dynorphin elicited dose-dependent analgesic effect in the range of 2.3--18.6 nmol. Calcu-lating on a molar basis of dynorphin was 6--10 times more potent than morphine and 65--100 times more potent than morphiceptin, another mu opiate receptor agonist Dynorphinanalgesia was by reversed by intrathecal injection of anti-dynorphin IgG and par-tten reversed by naloxone. Acute tolerance to morphine analgesia did not affect the occu-rance of dynorphin analgesia, indicating the absence of cross tolerance between morphine and dynorphin. Evidence from different lines of approach suggests that dynorphin may bindwith kappa opiate receptors in the spinal cord to exert its analgesic effect