The aim of the present study was to investigate whether clonidine and morphine interact positively to produce analgesia against the low intensity tonic pain represented by the formalin model in rats. Sub-threshold doses of morphine (0.5 nig kg-1) and clonidine (0.025 ing kg-1) were found to elecit marked antinociceptive effects when co-administered intraperitoneally, 15 min prior to formalin challenge. Repeated administration of this combination for eight days did not exhibit any significant decay of this analgesic response, whilst morphine (2 mg kg-1) -induced analgesia, deteriorated after similar administration. Clonidine and morphine thus exhibit a supra-additive effect against low intensity pain with negligible potential for induction of tolerance. This finding may be relevant for the long term control of chronic pain in certain clinical conditions.