A novel bivalent opioid tetrapeptide, biphalin (Tyr-D-Ala-Gly-Phe-NH)2, was synthesized based on structure-activity relationships. The analgesic activity of biphalin was assessed in comparison to morphine in rats. Drugs were administered subcutaneously (s.c.), intravenously (i.v.) and intrathecally (i.t.). Tail flick and tail pinch were used as tests for analgesia. Biphalin s.c. showed negligible analgesic activity, but when given i.v. produced significant analgesia, although less potent than morphine via this route. In contrast, intrathecal biphalin was more potent than morphine. These results indicate that biphalin has intrinsic activity that is compromised by enzymatic degradation or redistribution in the periphery, properties that may render it useful in exploring analgesic actions of locally applied opioids in the periphery without the likelihood of unwanted central effects.