Recently, an experimental model of monoarthritis was described in the rat induced by injection with Freund's adjuvant of the tibio-tarsal joint of one hindlimb. After injection, the clinical and behavioural signs of arthritis are stable from weeks 2 to 6 post-injection. Our purpose was to study the regulation of mu-, delta- and kappa-opioid binding sites in the superficial layers (laminae I-II) of the lumbar and cervical enlargements of the spinal cord 2, 4 and 6 weeks post-injection. Using quantitative receptor autoradiography and highly selective opioid ligands, we found complex changes consisting of a bilateral increase in specific [3H]DAMGO (Tyr*-D-Ala-Gly-NMe-Phe-Gly-ol) and [3H]pCl-DPDPE (Tyr*-D-Pen-Gly-Cl-Phe-D-Pen) binding at 2 weeks post-injection and a bilateral decrease in [3H]U-69593 ((5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1- oxaspiro(4,5)dec-8-yl]) specific binding at 4 weeks post-injection. These changes were restricted to the lumbar level. At 6 weeks post-injection, there was a bilateral increase in [3H]pCl-DPDPE specific binding at both lumbar and cervical levels. Altogether, these results suggest that, after probable local changes in endogenous opioid peptides, the three types of opioid binding sites are differentially involved in the development of the pathological process. These results contrast with the lack of significant modification in mu-, delta- and kappa-opioid binding classically reported at various levels of the spinal cord in polyarthritic rats at 3 weeks post-injection and verified for 2, 4 and 6 weeks post-injection in the present study.