Cysteine proteinase found in the spinal cord of rat, called nociceptin-converting enzyme (NCE), is competitively inhibited by dynorphin A and its fragment des-[Tyr(1)]-DYN A. This proteinase converts orphanin FQ/nociceptin (OFQ/N) to two major fragments: OFQ/N(1-11) and further OFQ/N(1-6) with analgesic properties. Dynorphin A at the concentration of 10 muM increases K-M from 15.0 to 55.9 muM. The calculated K-i for this interaction was estimated at 3.7 muM. This observation may suggest an interaction between opioid and nociceptive systems which may be affected by the balance between opioid and antiopioid systems. This balance between particular OFQ/N sequences that are derived from the same precursor and regulated by proteinases may play an important role in pain. Interestingly, dynorphin B does not reveal a similar action on the NCE.