We studied behavioral effects of the intraventricularly and intrathecally administered guanidinoethylmercaptosuccinic acid (GEMSA) - a potent inhibitor of enkephalin convertase. When given intraventricularly in doses of 3 and 6 micrograms, GEMSA elicited analgesia; after doses of 12.5 and 25 micrograms the explosive motor behavior and convulsions occurred. Following the intrathecal administration of GEMSA (12.5, 25 and 50 micrograms), an increase in the tail-flick latency was observed; moreover that drug potentiated analgesic effects of the intrathecally applied Met5-enkephalin-Arg6-Phe7 and Met5-enkephalin-Arg6-Gly7-Leu8. All the above effects of GEMSA were significantly attenuated by naloxone. The rats subjected to chronic pain showed a weaker analgesic response to the intrathecally injected GEMSA. The 3H-GEMSA binding to enkephalin convertase in the spinal cord of these rats produced only a slight increase in KD; besides, no changes in the enzyme activity were observed. The study shows that GEMSA has a potent pharmacological action in the central nervous system. Furthermore, this effect is partly due to the influence of GEMSA on endogenous opioid peptide systems, possibly on proenkephalin A.