The inhibition of both phases of the formalin response by intrathecal (IT) clonidine and baclofen, given alone or in combination at a fixed dose ratio, was studied. Both drugs, at doses not affecting motor performance, produced a dose-dependent inhibition of phase 2 of the formalin test. The potency of baclofen and clonidine, defined by their ID50's for phase 2 of the formalin test, was 0.56 and 3.4 nmol, respectively. The combination ID50 of baclofen and clonidine, with the equieffective dose ratio of 1:6, was found to be statistically lower than the theoretical additive ID50. These data suggest that co-administration of alpha2-adrenoceptor or GABA(B) receptor agonists may prove therapeutically useful in treating chronic pain.