A series of novel 6-substituted-2(3H)-benzothiazolones were synthesized and studied as analgesic agents. Among these compounds, two of them were found to exhibit potent analgesic activity in several in vivo tests (acetic acid writhing, Koster, carrageenan and PGE2 hyperalgesia). In these tests the most active compound of this series, i.e. 6-benzoyl-2(3H)-benzothiazolone (4a) was found to be superior to acetylsalicylic acid and equivalent to glafenine. The present study allows to conclude that 4a represents a new type of antinociceptive agent acting in periphery by inhibiting the cyclo-oxygenase pathway and promoting the release of an opioid peptide.